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

[Volodymyr Sapsai via llvm-commits]

vsapsai wrote:

Do you have any data on how faster the build gets with the precompiled headers? I've done some not particularly thorough experiments with clang and curious what you were able to gain.

And it is useful to measure the difference in CMake processing. @pogo59 will timing a command `cmake -GNinja llvm-project/llvm` be sufficient to estimate before/after changes? My understanding is that it should be enough but maybe you have something else in mind.

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