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

[Ameer J via llvm-commits]

ameerj wrote:

> That sounds good even if we would end up with more modest improvements. At least to me it is an indication that it is worth pursuing.

Reworked the changes to be far less intrusive. Currently, I see about ~1.5x build time improvements.
Without PCH, a clean build of check-llvm with one TARGET_TO_BUILD enabled took ~12-13 mins on my machine, 
with PCH it's close to ~8mins.

> Is it an option to make `LLVM_ENABLE_PRECOMPILED_HEADERS` OFF by default

This will definitely be set to OFF by default before merging, I just want to see if build errors are uncovered by the CI build systems

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