[llvm] [CMake] Make precompiled headers opt-in for ccache builds (PR #141927)

[Aiden Grossman via llvm-commits]

https://github.com/boomanaiden154 commented:

Have you tested how this interacts with `CMAKE_C_COMPILER_LAUNCHER` and `CMAKE_CXX_COMPILER_LAUNCHER` rather than `LLVM_CCACHE_BUILD`? That might be a bit of a footgun if we end up changing how the premerge system works (eg https://discourse.llvm.org/t/llvm-ccache-build-is-deprecated/68431?u=boomanaiden154-1).

https://github.com/llvm/llvm-project/pull/141927