[modules] Fix marking `ObjCMethodDecl::isOverriding` when there are no overrides.

Incorrect `isOverriding` flag triggers the assertion
`!Overridden.empty()` in `ObjCMethodDecl::getOverriddenMethods` when a
method is marked as overriding but we cannot find any overrides.

When a method is declared in a category and defined in implementation,
we don't treat it as an override because it is the same method with
a separate declaration and a definition. But with modules we can find
a method declaration both in a modular category and a non-modular category
with different memory addresses. Thus we erroneously conclude the method
is overriding. Fix by comparing canonical declarations that are the same
for equal entities coming from different modules.

rdar://92845511

Differential Revision: https://reviews.llvm.org/D138630