[llvm] [CMAKE][llvm-libraries] Add Precompiled Headers option to improve build times (PR #91755)

[Ameer J via llvm-commits]

ameerj wrote:

> To assess the impact on others do you have any requirements for the community regarding the support of this build mode? 

I am not sure how feasible it is, but I think it would make sense to have the Windows CI bot run with PCH enabled.
This way, any conflicts can be caught before a PR is merged, and the Windows build bot gets a build time improvement.

> I see now. It makes sense and is actually pretty smart to test in available CI.

> but we don't support a sufficiently new CMake

Unfortunately I don't see PCH working in the CI runs (I would expect to see a `cmake_pch.cxx` file being built)
What version of CMake is running on the buildkite bots? PCH support in CMake was added in version 3.16.


https://github.com/llvm/llvm-project/pull/91755