[clang][modules] Account for non-affecting inputs in `ASTWriter`

In D106876, we stopped serializing module map files that didn't affect compilation of the current module.

However, since each `SourceLocation` is simply an offset into `SourceManager`'s global buffer of concatenated input files in, these need to be adjusted during serialization. Otherwise, they can incorrectly point after the buffer or into subsequent input file.

This patch starts adjusting `SourceLocation`s, `FileID`s and other `SourceManager` offsets in `ASTWriter`.

Reviewed By: dexonsmith

Differential Revision: https://reviews.llvm.org/D136624