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

[Volodymyr Sapsai via llvm-commits]

vsapsai wrote:

Sorry for the looong delay in continuing the conversation, other stuff came up. How do you test the benefit from having certain headers in the precompiled header? I was interested in comparing the build time when

you don't have generated headers in PCH and use PCH for more targets
vs
you can have generated headers in PCH but cannot use PCH for certain targets

I have also collected [how often different headers are included by various .cpp files](https://gist.github.com/vsapsai/96eb91823d9379441cfbebf0ba3c0848) to decide which files are worth adding to PCH.

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