[clang][modules] NFCI: Pragma diagnostic mappings: write/read `FileID` instead...

[clang][modules] NFCI: Pragma diagnostic mappings: write/read `FileID` instead of `SourceLocation` (#87427)

For pragma diagnostic mappings, we always write/read `SourceLocation`
with offset 0. This is equivalent to just writing a `FileID`, which is
exactly what this patch starts doing.

Originally reviewed here: https://reviews.llvm.org/D137213