Cannot build `obj.clangAST` when `LLVM_UNREACHABLE_OPTIMIZATIONS` are turned off

[MasterAwesome]

I'm trying to build clang in Release mode with LLVM_UNREACHABLE_OPTIMIZATIONS turned off; however, the file AttrImpl.cpp fails to compile. This happens in release/17.x branch and main.

cmake llvm \
    -DLLVM_ENABLE_PROJECTS=clang \
    -DCMAKE_BUILD_TYPE=Release \
    -DCMAKE_C_COMPILER=clang \
    -DCMAKE_CXX_COMPILER=clang++ \
    -DCMAKE_ASM_COMPILER=clang \
    -DLLVM_UNREACHABLE_OPTIMIZE=OFF \
    -B build
cd build/
make obj.clangAST

This seems due to the fact that on CMAKE_BUILD_TYPE=Release + LLVM_UNREACHABLE_OPTIMIZE=OFF leads to

llvm-project/llvm/include/llvm/Support/ErrorHandling.h

Lines 151 to 154 in 6cbf6f5

	do { \
	LLVM_BUILTIN_TRAP; \
	LLVM_BUILTIN_UNREACHABLE; \
	} while (false)

branch which causes the tblgen'd output (AttrImpl.inc) to not compile. All other variants work [(Release + LLVM_UNREACHABLE_OPTIMIZE=ON), (Debug + LLVM_UNREACHABLE_OPTIMIZE=[ON|OFF])]

This is bisected to clang-tblgen change b6d4d51#diff-58111c7c545672c3a67b1b31a37d4103b35d9c584a46c74b0be14cc37741491bR2642-R2646

Tested with both host toolchains clang 17.0.2 and gcc 13.2.0

For Rust specific issue: Rust defaults to LLVM_UNREACHABLE_OPTIMIZE=OFF for all LLVM builds and this causes a regression when LLVM is compiled in release mode. See rust-lang/rust#109373

[MasterAwesome]

I can contribute to this change, where can I read about the procedure abut these types of changes, do I contribute to main using a PR and request for backport onto release/17.x. Also how do I claim the issue?

[AaronBallman]

/cherry-pick bffbe9a

[llvmbot]

Failed to cherry-pick: bffbe9a

https://github.com/llvm/llvm-project/actions/runs/6423348224

Please manually backport the fix and push it to your github fork. Once this is done, please add a comment like this:

/branch <user>/<repo>/<branch>