"illegal" is an overloaded word and can distress people who aren't familiar with
the usage as being illegal with respect to the language definition, not the
actual law.
- Reuse existing logic to curry the signature type with the 'self' of the context (in addition we no longer use a MetatypeType for the 'self' of a static member as they don't have conflicting signatures with instance members anyway)
- Limit the fix for SR-7251 to Swift 5 mode
- Add tests for generic subscripts
