[NFC] Adding a message to indicate LLVM_CCACHE_BUILD's future deprecation

[qiongsiwu]

This PR adds a message to indicate that LLVM_CCACHE_BUILD will be deprecated and suggests using CMAKE_C_COMPILER_LAUNCHER and CMAKE_CXX_COMPILER_LAUNCHER.

[daltenty]

LGTM

(one thing I note with this ccache support appears directly in llvm/CMakeList.txt, that makes me think this doesn't work for runtimes. but I think that's out of scope for this PR)

[tru]

I don't this we should add more stuff to LLVM_ENABLE_CCACHE. I think people should use CMAKE_C/CXX_COMPILER_LAUNCHER instead.

[daltenty]

I don't this we should add more stuff to LLVM_ENABLE_CCACHE. I think people should use CMAKE_C/CXX_COMPILER_LAUNCHER instead.

Thanks for the tip. Reading the CMake docs that seems a reasonable solution.

Should we make mention of this in https://github.com/llvm/llvm-project/blob/main/llvm/docs/CMake.rst and/or issue a message about this option being deprecated? Those docs are what originally led us down this path.

[tru]

Yes - I would welcome some documentation updates that points out the upstream solution and that the LLVM_ option is mainly kept around as a legacy thing. I tried to remove it a while ago but encountered push back, so I would avoid saying it's totally deprecated.

cc @Endilll

[Endilll]

I'd like to second Tobias' feedback. CMAKE_<LANG>_COMPILER_LAUNCHER has been around since CMake 3.4, while our minimum requirement in 3.20, so there are no compatibility reasons to do this.

On top of that, I think compiler launcher should be treated as a part of toolchain. That is, if present, it should be either set by user on the command line, or in a toolchain file. Notably, we shouldn't try to be smart about it after project() is called, like unofficial (yet Kitware-hosted) documentation has been saying for at least 6 years about CMAKE_<LANG>_COMPILER: https://gitlab.kitware.com/cmake/community/-/wikis/FAQ#method-3-avoid-use-set

[qiongsiwu]

Thanks for your comments everyone! The PR is updated to only add a CMake message, and the documentation is updated to reflect our suggestion to use CMAKE_C_COMPILER_LAUNCHER and CMAKE_CXX_COMPILER_LAUNCHER.

[qiongsiwu]

Ping for more comments. If there are none, I will land this PR soon. Thanks!