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

[Ameer J via llvm-commits]

ameerj wrote:

CI seems to handle building PCH with no issue.

How do the maintainers feel about enabling PCH by default for MSVC and/or Windows?

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