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

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

I've seen this happen on [a pre-merge check for my own PR too](https://buildkite.com/llvm-project/github-pull-requests/builds/184094#01971875-3816-4cd5-a467-43ebd91f916d). No idea which part of the environment could be causing this, but maybe now that we know this works in general we can make it opt-in either for local developer setups to make use of or for specific build bots to try out.

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