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

[Ameer J via llvm-commits]

ameerj wrote:

@vsapsai The proposal is fair. I've disabled PCH by default, making it opt-in. 
So this PR is simply adding in PCH support that anyone can enable to try if they wish.

The CI failure seems unrelated to my changes.

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