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

[Aiden Grossman via llvm-commits]

boomanaiden154 wrote:

> Do you know what's different about the buildkite caching setup?

It's different from Github in that it's persistent across builds inside the same container. It could even be a single runner that's messed up somewhere.

Can we hold off on landing this until after we sunset Buildkite (should happen in the next couple days)? If it's still occurring on the Github side we can land it, but otherwise we probably don't need it.

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