swift234

Make typealiases in protocols available for nested type lookup.

This involved several complications, mostly making the distinction between typealiases whose definitions contain associated types and those which don't:
- Only check final component of compound idents for existential-typeness, so that it is legal to refer to a nested typealias inside the existential.
- New diagnosis for trying to use typealias which is an alias for an associated type outside of the defining protocol.
- Add check on member lookup to omit typealiases to assoc types, similar to existing treatment of assoc types.
- Added test.