[c++-interop] Teach APINotes to handle ObjCContainer items while in C++-Interop

[plotfi]

In a previous commit I taught APINotes to treat extern-c context as a
toplevel file context, but this only makes sense when we are handling
the decls handled in the block like VarDecls, FunctionDecls,
ObjCInterfaceDecls, etc.

But when the decl being handled for APINote is an ObjCMethod inside of a
ObjCContainer then it is important to allow the subsequent code in the
Sema::ProcessAPINotes function to handle things.

This can be excercised by the following APINote:

---
Name: SomeModule
Classes:
- Name: NSSomeClass
  SwiftName: SomeClass
  Methods:
  - Selector: 'didMoveToParentViewController:'
    SwiftName: didMove(toParent:)
    MethodKind: Instance
    

[plotfi]

@hyp @zoecarver @bulbazord @drodriguez

Test is coming, but this should fix the ObjCMethod case. I will have a PR for the swift repo as well soon. Basically this covers:

// test.apinote:

---
Name: SomeModule
Classes:
- Name: NSSomeClass
  SwiftName: SomeClass
  Methods:
  - Selector: 'didMoveToParentViewController:'
    SwiftName: didMove(toParent:)
    MethodKind: Instance

// test.h
@interface NSSomeClass
  -(instancetype)init;
@end

@interface NSSomeClass (SomeCategory)
- (void)didMoveToParentViewController:(NSSomeClass *)parent;
@end

[drodriguez]

I did a quick check and it seems to remove 50 failing tests from the full test suite with C++ interop enabled (down to 452 from 502). Seems promising.

[hyp]

@swift-ci please test

[zoecarver]

@swift-ci please test

[plotfi]

With #4148 already landed, this commit is just an added test case for ObjCMethod.

[hyp]

@swift-ci please test