[ASTStructuralEquivalence] Fix crash when ObjCCategoryDecl doesn't have...

[ASTStructuralEquivalence] Fix crash when ObjCCategoryDecl doesn't have corresponding ObjCInterfaceDecl.

When this happens, it is invalid code and there is diagnostic
```
error: cannot find interface declaration for '...'
```

But clang shouldn't crash even if code is invalid. Though subsequent
diagnostic can be imperfect because without ObjCInterfaceDecl we don't have
a type for error messages.

rdar://108818430

Differential Revision: https://reviews.llvm.org/D151523