[APINotes] Support annotating C++ methods

This adds support for adding Clang attributes to C++ methods declared
within C++ records by using API Notes.

For instance:
```
Tags:
- Name: IntWrapper
  Methods:
  - Name: getIncremented
    Availability: none
```

This is the first instance of something within a C++ record being
annotated with API Notes, so it adds the necessary infra to make a C++
record an "API Notes context".

Notably this does not add support for nested C++ tags. That will be
added in a follow-up patch.

rdar://131387880

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