tree-wide: port LDC to LLVM 18

[liushuyu]

This pull request adds LLVM 18.1.x support to LDC.
LLVM recently switched to a new versioning scheme, where LLVM 18.1.0 was the first stable release of LLVM 18 series.

Aside from the usual API renaming, zext can not be used directly now and needs to be utilized through a constant folding mechanism.

[thewilsonator]

If you could add profdata and Filecheck (as done in this PR at the very bottom), that would be great

[liushuyu]

If you could add profdata and Filecheck (as done in this PR at the very bottom), that would be great

Will do. I cannot add LLVM 18 CI at the moment due to LLVM upstream missing binary packages for arm64 macOS.
I will open a separate pull request to address the CI issue.

[liushuyu]

If you could add profdata and Filecheck (as done in this PR at the very bottom), that would be great

@thewilsonator Done. Added files from LLVM upstream source.

[kinke]

Thx a lot, much appreciated!

[kinke]

This could probably be simplified to something like val = DtoConstUbyte(val->isNullValue() ? 0 : 1);.

[liushuyu]

Replaced.

[kinke]

@JohanEngelen: I just remembered #4559 - I guess this might be dangerous here too?

[JohanEngelen]

yeah, good point. At the very least, this change should be in its own PR, with careful checking.
Keep this (big) PR about LLVM18 compatibility only.

[liushuyu]

Should I just revert the changes to use LLVM constant folding here?

[kinke]

Yeah please, and sorry about the wrong hint.

[liushuyu]

Yeah please, and sorry about the wrong hint.

No problem, just a partial revert away. Now pushed.

[kinke]

There's also

ldc/cmake/Modules/FindLLVM.cmake

Lines 30 to 63 in ef72ded

	# Try suffixed versions to pick up the newest LLVM install available on Debian
	# derivatives.
	# We also want an user-specified LLVM_ROOT_DIR to take precedence over the
	# system default locations such as /usr/local/bin. Executing find_program()
	# multiples times is the approach recommended in the docs.
	set(llvm_config_names llvm-config-17.0 llvm-config170 llvm-config-17
	llvm-config-16.0 llvm-config160 llvm-config-16
	llvm-config-15.0 llvm-config150 llvm-config-15
	llvm-config-14.0 llvm-config140 llvm-config-14
	llvm-config-13.0 llvm-config130 llvm-config-13
	llvm-config-12.0 llvm-config120 llvm-config-12
	llvm-config-11.0 llvm-config110 llvm-config-11
	llvm-config)
	find_program(LLVM_CONFIG
	NAMES ${llvm_config_names}
	PATHS ${LLVM_ROOT_DIR}/bin NO_DEFAULT_PATH
	DOC "Path to llvm-config tool.")
	find_program(LLVM_CONFIG NAMES ${llvm_config_names})
	if(APPLE)
	# extra fallbacks for MacPorts & Homebrew
	find_program(LLVM_CONFIG
	NAMES ${llvm_config_names}
	PATHS /opt/local/libexec/llvm-17/bin
	/opt/local/libexec/llvm-16/bin /opt/local/libexec/llvm-15/bin
	/opt/local/libexec/llvm-14/bin /opt/local/libexec/llvm-13/bin
	/opt/local/libexec/llvm-12/bin /opt/local/libexec/llvm-11/bin
	/opt/local/libexec/llvm/bin
	/usr/local/opt/llvm@17/bin
	/usr/local/opt/llvm@16/bin /usr/local/opt/llvm@15/bin
	/usr/local/opt/llvm@14/bin /usr/local/opt/llvm@13/bin
	/usr/local/opt/llvm@12/bin /usr/local/opt/llvm@11/bin
	/usr/local/opt/llvm/bin
	NO_DEFAULT_PATH)
	endif()

where we try to find a matching llvm-config* executable.

[liushuyu]

CI failures seems unrelated and can be retried

[liushuyu]

https://github.com/ldc-developers/ldc/actions/runs/8410785498/job/23029649005?pr=4599 This CI job needs a retry.

[kinke]

[Following up with some fixes & additions in #4604.]