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

[Volodymyr Sapsai via llvm-commits]

vsapsai wrote:

Ok, looks like there are no objections to the usage of precompiled headers in general. So now I'll look at the technical side of the change. Please note that I'm not good with CMake and might ask some stupid questions.

I've tried your patch locally and hitting the error
```
-- LLVM Lib Precompiled Headers are enabled
CMake Error at cmake/modules/LLVMPrecompiledHeaders.cmake:2 (get_property):
  get_property DIRECTORY scope provided but requested directory was not
  found.  This could be because the directory argument was invalid or, it is
  valid but has not been processed yet.
Call Stack (most recent call first):
  cmake/modules/LLVMPrecompiledHeaders.cmake:13 (get_all_targets_recursive)
  cmake/modules/LLVMPrecompiledHeaders.cmake:25 (get_all_targets)
  cmake/modules/LLVMPrecompiledHeaders.cmake:69 (add_llvm_subdir_pch)
  CMakeLists.txt:1419 (llvm_lib_precompiled_headers)
```

I haven't investigated the issue yet but maybe it is something you've seen, so I've decided to check first if it is a known problem.

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