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

[Volodymyr Sapsai via llvm-commits]

vsapsai wrote:

> I believe the issue may have been case sensitivity with some paths, should be resolved with the latest commit.

Thanks, that fixes the issue.

Do you want to make the precompiled headers for CI build or for incremental local development? Asking because it affects how much to add to the precompiled header. For example, having in a pch multiple often-modified headers can slow down the incremental builds but it doesn't have the same downsides for clean builds.

Have you thought about dependencies between PCH and generated headers? For example, should we use Attributes.inc in the precompiled header? If yes, does it mean that PCH shouldn't be used for llvmIR? Personally I'm not excited about managing layering between PCH and various libraries but I'm curious to know your opinion.

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