[ASTImporter] Fix import of ObjCPropertyDecl that share the same name

Objective-C apparently allows name conflicts between instance and class
properties, so this is valid code:

```
@protocol DupProp
@property (class, readonly) int prop;
@property (readonly) int prop;
@end
```

The ASTImporter however isn't aware of this and will consider the two properties
as if they are the same property because it just compares their name and types.
This causes that when importing both properties we only end up with one property
(whatever is imported first from what I can see).

Beside generating a different AST this also leads to a bunch of asserts and
crashes as we still correctly import the two different getters for both
properties (the import code for methods does the correct check where it
differentiated between instance and class methods). As one of the setters will
not have its associated ObjCPropertyDecl imported, any call to
`ObjCMethodDecl::findPropertyDecl` will just lead to an assert or crash.

Fixes rdar://74322659

Reviewed By: shafik, kastiglione

Differential Revision: https://reviews.llvm.org/D99077