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

[Michael Kruse via llvm-commits]

Meinersbur wrote:

> So this is an actual size mismatch. If the header in question has actually changed, then CMake should have rebuilt it prior to rebuilding its users and this should not happen. But somehow it seems to irregularly pop up on some build bots. I am yet to see it on a local build, though that doesn't mean nobody else has.

Sounds like a problem of dependencies not being honored. So this is just a workaround for another a different problem?

In any case, I find the summary and source comment insufficient. If this is indeed a workaround for an yet unknown cause, please also link to the issue # (#142449 ?) and add a `FIXME` comment. The fault may also lie in ccache not restoring the right .pch files before invoking clang.

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