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

[Tobias Hieta via llvm-commits]

tru wrote:

> My knee-jerk reaction would be: Why don't you just use modules, since we are maintaining a complete module map for the project? Is this an MSVC-specific option?

I don't think modules are supported when compiling with clang-cl on windows, if I remember correctly. In that case this would also speed up that config and not just MSVC. 

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