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

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

> Also, has the issue been observed at all on the Github hosted runners? They use a different caching setup and I've only seen reports on the Buildkite side. None of the issues have links to Github workflows but that doesn't necessarily indicate anything.

I've only seen the issue on Buildkite as well. It doesn't seem to be happening on the buildbots because the flang builds have been passing pretty consistently lately, but maybe I just haven't noticed.

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