[modules] While merging ObjCInterfaceDecl definitions, merge them as decl contexts too.

While working on https://reviews.llvm.org/D110280 I've tried to merge
decl contexts as it seems to be correct and matching our handling of
decl contexts from different modules. It's not required for the fix in
https://reviews.llvm.org/D110280 but it revealed a missing diagnostic,
so separating this change into a separate commit.

Renamed some variables to distinguish diagnostic like "declaration of
'x' does not match" for different cases.

Differential Revision: https://reviews.llvm.org/D110287