[ODRHash] Detect duplicate `ObjCProtocolDecl` ODR mismatches during parsing.

When during parsing we encountered a duplicate `ObjCProtocolDecl`, we
were always emitting an error. With this change we accept
* when a previous `ObjCProtocolDecl` is in a hidden [sub]module;
* parsed `ObjCProtocolDecl` is the same as the previous one.

And in case of mismatches we provide more detailed error messages.

rdar://93069080

Differential Revision: https://reviews.llvm.org/D130327