[llvm] [NFC] Adding a message to indicate LLVM_CCACHE_BUILD's future deprecation (PR #90624)

[Qiongsi Wu via llvm-commits]

qiongsiwu wrote:

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`. 

https://github.com/llvm/llvm-project/pull/90624