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

[Jakub Kuderski via llvm-commits]

kuhar wrote:

> > Why don't you just use modules?
> 
> Looks like CMake 3.28 introduces support for [cxxmodules](https://cmake.org/cmake/help/latest/manual/cmake-cxxmodules.7.html).

I think the suggestion was to try clang modules instead of c++ modules.

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