Sema: Test inheritance clause references to nested type of class-constrained archetype

This hits the FIXME case in CompleteGenericTypeResolver's
resolveDependentMemberType() method I thought was dead, but
it's actually not.

Author: slavapestov

test/Generics/superclass_constraint.swift

@@ -136,3 +136,19 @@ func superclassConformance4<T: P3, U: P3>(_: T, _: U)
   where T.T: C, // expected-note{{superclass constraint 'T.T' : 'C' written here}}
         U.T: C, // expected-warning{{redundant superclass constraint 'U.T' : 'C'}}
         T.T == U.T { }
+
+// Lookup of superclass associated types from inheritance clause
+
+protocol Elementary {
+  associatedtype Element
+
+  func get() -> Element
+}
+
+class Classical : Elementary {
+  func get() -> Int {
+    return 0
+  }
+}
+
+func genericFunc<T : Elementary, U : Classical>(_: T, _: U) where T.Element == U.Element {}