…xt, as long as they don't depend on 'Self'

Suppose you have this protocol:

protocol P {
  typealias A = Int
  typealias B = Self
}

Clearly, 'P.B' does not make sense, because then the type parameter
would "leak out" of the existential container. However, 'P.A' is totally
fine, it just means 'Int'.

Previously, we would allow 'P.A' in type context, and diagnose on 'P.B'.

However, due to an oversight, neither one was allowed in expression
context, so for example you could not write 'P.A.self', even though
that should just mean 'Int.self'.

Fix this by generalizing performMemberLookup(), and fix up some
diagnostics to be more specific when something is wrong -- we want
to avoid talking about typealiases as 'static members', since that
doesn't really make much sense.

Fixes <https://bugs.swift.org/browse/SR-2314>.