[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
