[SymbolGraph] Ignore some Self requirements
To ease the burden on the client, ignore some generic requirements involving
Self. For example, `Self: P` where we already know that `Self` conforms to `P`.
An example case:
```
public struct S: Equatable {
public static func ==(lhs: S, rhs: S) -> Bool { ... }
}
```
`!=` is defined in terms of `Self` and the default implementation has a `Self:
Equatable`. For the purposes of documentation, it's not necessary to specify
that again on the page of documentation for `!=`.
rdar://60963924
