[Frontend] Restore Preprocessor::getPredefines()

https://reviews.llvm.org/rG6bbf51f3ed59ae37f0fec729f25af002111c9e74 from May removed Preprocessor::getPredefines() from Clang's API, presumably as a cleanup because this method is unused in the LLVM codebase.

However, it was/is used by a small number of third-party tools and is pretty harmless, so this patch adds it back and documents why it's here.

The issue was raised in https://github.com/llvm/llvm-project/issues/57483, it would be nice to be able to land it into Clang 15 as it breaks those third-party tools and we can't easily add it back in bug fix releases.

Reviewed By: brad.king, thieta

Differential Revision: https://reviews.llvm.org/D133044