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

[Volodymyr Sapsai via llvm-commits]

vsapsai wrote:

> With a very hacked up PCH implementation I was able to get close to 1.8x build time improvements.

That sounds good even if we would end up with more modest improvements. At least to me it is an indication that it is worth pursuing.

Is it an option to make `LLVM_ENABLE_PRECOMPILED_HEADERS` OFF by default? So that it won't affect everyone but only people who decided to opt in.

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