[X86] avx512fp16intrin.h - allow fp16 truncation casts to be used in constexpr

[RKSimon]

_mm256_castph256_ph128
_mm512_castph512_ph128
_mm512_castph512_ph256

Part of #156348

[llvm-ci]

LLVM Buildbot has detected a new failure on builder lldb-arm-ubuntu running on linaro-lldb-arm-ubuntu while building clang at step 6 "test".

Full details are available at: https://lab.llvm.org/buildbot/#/builders/18/builds/21780

Here is the relevant piece of the build log for the reference

Step 6 (test) failure: build (failure)
...
PASS: lldb-api :: tools/lldb-dap/moduleSymbols/TestDAP_moduleSymbols.py (1225 of 3716)
PASS: lldb-api :: tools/lldb-dap/evaluate/TestDAP_evaluate.py (1226 of 3716)
PASS: lldb-api :: tools/lldb-dap/optimized/TestDAP_optimized.py (1227 of 3716)
PASS: lldb-api :: tools/lldb-dap/output/TestDAP_output.py (1228 of 3716)
PASS: lldb-api :: tools/lldb-dap/repl-mode/TestDAP_repl_mode_detection.py (1229 of 3716)
PASS: lldb-api :: tools/lldb-dap/launch/TestDAP_launch.py (1230 of 3716)
UNSUPPORTED: lldb-api :: tools/lldb-dap/restart/TestDAP_restart_console.py (1231 of 3716)
UNSUPPORTED: lldb-api :: tools/lldb-dap/runInTerminal/TestDAP_runInTerminal.py (1232 of 3716)
UNSUPPORTED: lldb-api :: tools/lldb-dap/save-core/TestDAP_save_core.py (1233 of 3716)
PASS: lldb-api :: tools/lldb-dap/restart/TestDAP_restart.py (1234 of 3716)
FAIL: lldb-api :: tools/lldb-dap/progress/TestDAP_Progress.py (1235 of 3716)
******************** TEST 'lldb-api :: tools/lldb-dap/progress/TestDAP_Progress.py' FAILED ********************
Script:
--
/usr/bin/python3.10 /home/tcwg-buildbot/worker/lldb-arm-ubuntu/llvm-project/lldb/test/API/dotest.py -u CXXFLAGS -u CFLAGS --env LLVM_LIBS_DIR=/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./lib --env LLVM_INCLUDE_DIR=/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/include --env LLVM_TOOLS_DIR=/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./bin --arch armv8l --build-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build.noindex --lldb-module-cache-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build.noindex/module-cache-lldb/lldb-api --clang-module-cache-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build.noindex/module-cache-clang/lldb-api --executable /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./bin/lldb --compiler /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./bin/clang --dsymutil /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./bin/dsymutil --make /usr/bin/gmake --llvm-tools-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./bin --lldb-obj-root /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/tools/lldb --lldb-libs-dir /home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/./lib --cmake-build-type Release /home/tcwg-buildbot/worker/lldb-arm-ubuntu/llvm-project/lldb/test/API/tools/lldb-dap/progress -p TestDAP_Progress.py
--
Exit Code: 1

Command Output (stdout):
--
lldb version 22.0.0git (https://github.com/llvm/llvm-project.git revision b1788211d3750292fc10027fb649b359fd6cd0d7)
  clang revision b1788211d3750292fc10027fb649b359fd6cd0d7
  llvm revision b1788211d3750292fc10027fb649b359fd6cd0d7
Skipping the following test categories: ['libc++', 'msvcstl', 'dsym', 'gmodules', 'debugserver', 'objc']

--
Command Output (stderr):
--
========= DEBUG ADAPTER PROTOCOL LOGS =========
1759835127.161489010 (stdio) --> {"command":"initialize","type":"request","arguments":{"adapterID":"lldb-native","clientID":"vscode","columnsStartAt1":true,"linesStartAt1":true,"locale":"en-us","pathFormat":"path","supportsRunInTerminalRequest":true,"supportsVariablePaging":true,"supportsVariableType":true,"supportsStartDebuggingRequest":true,"supportsProgressReporting":true,"supportsInvalidatedEvent":true,"supportsMemoryEvent":true,"$__lldb_sourceInitFile":false},"seq":1}
1759835127.162044764 (stdio) queued (command=initialize seq=1)
1759835127.166735411 (stdio) <-- {"body":{"$__lldb_version":"lldb version 22.0.0git (https://github.com/llvm/llvm-project.git revision b1788211d3750292fc10027fb649b359fd6cd0d7)\n  clang revision b1788211d3750292fc10027fb649b359fd6cd0d7\n  llvm revision b1788211d3750292fc10027fb649b359fd6cd0d7","completionTriggerCharacters":["."," ","\t"],"exceptionBreakpointFilters":[{"description":"C++ Catch","filter":"cpp_catch","label":"C++ Catch","supportsCondition":true},{"description":"C++ Throw","filter":"cpp_throw","label":"C++ Throw","supportsCondition":true},{"description":"Objective-C Catch","filter":"objc_catch","label":"Objective-C Catch","supportsCondition":true},{"description":"Objective-C Throw","filter":"objc_throw","label":"Objective-C Throw","supportsCondition":true}],"supportTerminateDebuggee":true,"supportsBreakpointLocationsRequest":true,"supportsCancelRequest":true,"supportsCompletionsRequest":true,"supportsConditionalBreakpoints":true,"supportsConfigurationDoneRequest":true,"supportsDataBreakpoints":true,"supportsDelayedStackTraceLoading":true,"supportsDisassembleRequest":true,"supportsEvaluateForHovers":true,"supportsExceptionFilterOptions":true,"supportsExceptionInfoRequest":true,"supportsFunctionBreakpoints":true,"supportsHitConditionalBreakpoints":true,"supportsInstructionBreakpoints":true,"supportsLogPoints":true,"supportsModuleSymbolsRequest":true,"supportsModulesRequest":true,"supportsReadMemoryRequest":true,"supportsSetVariable":true,"supportsSteppingGranularity":true,"supportsValueFormattingOptions":true,"supportsWriteMemoryRequest":true},"command":"initialize","request_seq":1,"seq":0,"success":true,"type":"response"}
1759835127.167524576 (stdio) --> {"command":"launch","type":"request","arguments":{"program":"/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build.noindex/tools/lldb-dap/progress/TestDAP_Progress.test/a.out","stopOnEntry":true,"initCommands":["settings clear --all","settings set symbols.enable-external-lookup false","settings set target.inherit-tcc true","settings set target.disable-aslr false","settings set target.detach-on-error false","settings set target.auto-apply-fixits false","settings set plugin.process.gdb-remote.packet-timeout 60","settings set symbols.clang-modules-cache-path \"/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build.noindex/module-cache-lldb/lldb-api\"","settings set use-color false","settings set show-statusline false"],"disableASLR":false,"enableAutoVariableSummaries":false,"enableSyntheticChildDebugging":false,"displayExtendedBacktrace":false},"seq":2}
1759835127.167614460 (stdio) queued (command=launch seq=2)
1759835127.168050289 (stdio) <-- {"body":{"category":"console","output":"Running initCommands:\n"},"event":"output","seq":0,"type":"event"}
1759835127.168394804 (stdio) <-- {"body":{"category":"console","output":"(lldb) settings clear --all\n"},"event":"output","seq":0,"type":"event"}
1759835127.168517113 (stdio) <-- {"body":{"category":"console","output":"(lldb) settings set symbols.enable-external-lookup false\n"},"event":"output","seq":0,"type":"event"}
1759835127.168623924 (stdio) <-- {"body":{"category":"console","output":"(lldb) settings set target.inherit-tcc true\n"},"event":"output","seq":0,"type":"event"}
1759835127.168707609 (stdio) <-- {"body":{"category":"console","output":"(lldb) settings set target.disable-aslr false\n"},"event":"output","seq":0,"type":"event"}
1759835127.168788195 (stdio) <-- {"body":{"category":"console","output":"(lldb) settings set target.detach-on-error false\n"},"event":"output","seq":0,"type":"event"}
1759835127.168912649 (stdio) <-- {"body":{"category":"console","output":"(lldb) settings set target.auto-apply-fixits false\n"},"event":"output","seq":0,"type":"event"}
1759835127.168950796 (stdio) <-- {"body":{"category":"console","output":"(lldb) settings set plugin.process.gdb-remote.packet-timeout 60\n"},"event":"output","seq":0,"type":"event"}
1759835127.168967009 (stdio) <-- {"body":{"category":"console","output":"(lldb) settings set symbols.clang-modules-cache-path \"/home/tcwg-buildbot/worker/lldb-arm-ubuntu/build/lldb-test-build.noindex/module-cache-lldb/lldb-api\"\n"},"event":"output","seq":0,"type":"event"}
1759835127.168980598 (stdio) <-- {"body":{"category":"console","output":"(lldb) settings set use-color false\n"},"event":"output","seq":0,"type":"event"}
1759835127.168993711 (stdio) <-- {"body":{"category":"console","output":"(lldb) settings set show-statusline false\n"},"event":"output","seq":0,"type":"event"}
1759835127.410531759 (stdio) <-- {"body":{"percentage":100,"progressId":"5","timestamp":"1759835127.199918747"},"event":"progressEnd","seq":0,"type":"event"}
1759835127.499696493 (stdio) <-- {"command":"launch","request_seq":2,"seq":0,"success":true,"type":"response"}
1759835127.499809980 (stdio) <-- {"event":"initialized","seq":0,"type":"event"}
1759835127.501187563 (stdio) --> {"command":"threads","type":"request","arguments":{},"seq":3}