Instead of emitting a redefinition error, check that definitions are
equivalent and allow such scenario.
A few non-obvious implementation details:
* to avoid multiple definitions in the redeclaration chain we just drop
the new definition after checking for equivalence;
* for checking definition equivalence use ODR hash instead of
ASTStructuralEquivalence because it avoids excessive recursive
deserialization. Though after detecting a mismatch we do deserialize
multiple entities to provide a better error message.
rdar://82908223
Differential Revision: https://reviews.llvm.org/D124286
When two modules contain interfaces with the same name, check the
definitions are equivalent and diagnose if they are not.
Differential Revision: https://reviews.llvm.org/D140073
