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

[Kajetan Puchalski via llvm-commits]

mrkajetanp wrote:

> So this is just a workaround for another a different problem?

Exactly. It doesn't seem to happen very consistently, otherwise the CI would have been failing a lot more than it has lately. It seems like there's a bug either in CMake or ccache causing this interaction between the two, unless it's some other environment thing I'm not considering. If we make it opt-in this way, then it should be easier to pinpoint the issue over time because people configuring build bots can enable it and then see whether this ends up happening for them or not.

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