clang-tblgen build: avoid duplicate inclusion of libLLVMSupport

TableGen executables are supposed to never be linked against libLLVM-*.so,
even when LLVM_LINK_LLVM_DYLIB=ON, presumably for cross-compilation.

It turns out that clang-tblgen *did* link against libLLVM-*.so,
indirectly so via the clangSupport.

This lead to a regression in what should have been unrelated work of
cleaning up ManagedStatics in LLVMSupport. A running clang-tblgen
process ended up with two copies of a cl::opt static global:

- one from libLLVMSupport linked statically into clang-tblgen as a
  direct dependency
- one from libLLVMSupport linked into libLLVM-*.so, which clang-tblgen
  linked against due to the clangSupport dependency

For a bit more context, see the discussion at
https://discourse.llvm.org/t/flang-aarch64-dylib-buildbot-need-help-understanding-a-regression-in-clang-tblgen/64871/

None of the potential solutions I could find are perfect. Presumably one
possible solution would be to remove "Support" from the explicit
dependencies of clang-tblgen. However, relying on the transitive
inclusion via clangSupport seems risky, and in any case this wouldn't
address the issue of clang-tblgen surprisingly linking against libLLVM-*.so.

This change instead creates a second version of the clangSupport library
that is explicitly linked statically, to be used by clang-tblgen.

v2:
- define an alias so that clang-tblgen can always link against
  "clangSupport_tablegen"
- use add_llvm_library(... BUILDTREE_ONLY ...)

v3:
- use the object library version of clangSupport if available

Differential Revision: https://reviews.llvm.org/D134637

clang/lib/Support/CMakeLists.txt

@@ -9,8 +9,24 @@ set(LLVM_LINK_COMPONENTS
   Support
   )
 
-add_clang_library(clangSupport
+set(clangSupport_sources
   RISCVVIntrinsicUtils.cpp
   )
 
+add_clang_library(clangSupport ${clangSupport_sources})
+
+if (NOT XCODE)
+  add_library(clangSupport_tablegen ALIAS obj.clangSupport)
+elseif (NOT LLVM_LINK_LLVM_DYLIB)
+  add_library(clangSupport_tablegen ALIAS clangSupport)
+else()
+  # Build a version of the support library that does not link against
+  # libLLVM-*.so, to be used by clang-tblgen. This is so clang-tblgen doesn't
+  # link against libLLVMSupport twice (once statically and once via
+  # libLLVM-*.so).
+  add_llvm_library(clangSupport_tablegen
+    BUILDTREE_ONLY STATIC DISABLE_LLVM_LINK_LLVM_DYLIB
+    ${clangSupport_sources})
+endif()
+
 set(LLVM_COMMON_DEPENDS ${LLVM_COMMON_DEPENDS_OLD})

clang/utils/TableGen/CMakeLists.txt

@@ -25,6 +25,6 @@ add_tablegen(clang-tblgen CLANG
   TableGen.cpp
   )
 
-target_link_libraries(clang-tblgen PRIVATE clangSupport)
+target_link_libraries(clang-tblgen PRIVATE clangSupport_tablegen)
 
 set_target_properties(clang-tblgen PROPERTIES FOLDER "Clang tablegenning")