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

[Paul T Robinson via llvm-commits]

pogo59 wrote:

You probably should post an RFC on [discourse](https://discourse.llvm.org/) for wider visibility, as this affects essentially everyone.

It looks like PCH is strictly an MSVC thing? but the change in llvm/CMakeLists.txt is executed for all builds, which slows down CMake processing for everyone.

I see a number of formatting changes in this PR. Probably you ran clang-format on the modified files? We usually don't want to mix formatting changes with functional changes. You should use clang-format-diff instead.

The new header does not follow the coding standards, see how we like to do [file headers](https://llvm.org/docs/CodingStandards.html#file-headers) and [header guards](https://llvm.org/docs/CodingStandards.html#header-guard).

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