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

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

> 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).

If LLVM_CCACHE_BUILD gets removed then I think that whole part of this file will have to get reworked anyway. The last time I was making changes around ccache I was told that enabling ccache through the `*LAUNCHER` options is not supposed to be fully supported in the same way as `LLVM_CCACHE_BUILD`. As in, the former is supposed to work, but not proactively set things up for the user in the same way as the latter.

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