Sema: Fix type lookup from protocol extensions with a class-constrained 'Self'

A protocol extension can add additional generic constraints on
'Self' or associated types thereof. In particular, 'Self' itself
can have a superclass constraint placed on it.

There were a couple of problems with this corner case:

- Type aliases defined in protocols that 'Self' conforms to _as a
  concrete type_ to were not handled properly, triggering an assertion.

  For example,

  protocol P { typealias T = ... }
  class C : P {}
  protocol Q {}

  extension Q where Self : C { ... T ... }

  The conformance o P comes from the 'Self : C' constraint.

- If the type was found in a superclass of 'Self', we used the wrong
  base type for the substitution.

  For example,

  protocol P {}
  class C<T> { typealias A = T }
  class D : C<Int> {}

  extension P where Self : D { ... A ... }

  The substituted type of 'A' should be computed with a self type
  of C<Int> here.

Also, take another stab at cleaning up the mess that is
resolveTypeInContext() and related bits of code.

Author: slavapestov