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

[Volodymyr Sapsai via llvm-commits]

vsapsai wrote:

> > Is it an option to make `LLVM_ENABLE_PRECOMPILED_HEADERS` OFF by default
> 
> This will definitely be set to OFF by default before merging, I just want to see if build errors are uncovered by the CI build systems

I see now. It makes sense and is actually pretty smart to test in available CI.

To assess the impact on others do you have any requirements for the community regarding the support of this build mode? For example, if after a commit the PCH build fails, is it OK if there is no prompt revert but you need to figure out what is broken and propose a fix? In the very beginning you've mentioned
> The aim is to be as non-intrusive as possible with the addition.

so I assume that you don't expect any maintenance contributions from the rest of the community.

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