[clang][modules][pch] Allow loading PCH with different modules cache path

It's useful to be able to load explicitly-built PCH files into an implicit build (e.g. during dependency scanning). That's currently impossible, since the explicitly-built PCH has an empty modules cache path, while the current compilation has (and needs to have) a valid path, triggering an error in the `PCHValidator`.

This patch adds a preprocessor option and command-line flag that can be used to omit this check.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D103802