[ODRHash] Hash `RecordDecl` and diagnose discovered mismatches.

When two modules contain struct/union with the same name, check the
definitions are equivalent and diagnose if they are not. This is similar
to `CXXRecordDecl` where we already discover and diagnose mismatches.

rdar://problem/56764293

Differential Revision: https://reviews.llvm.org/D71734