Check type witnesses in a couple of associated type inference tests

Author: slavapestov

test/decl/protocol/req/associated_type_inference_tautology_2.swift

@@ -18,3 +18,5 @@ struct S: P {
   func f(_: Float) {}
   func f(_: String) {}
 }
+
+let x: String.Type = S.A.self

test/decl/protocol/req/rdar122586685.swift

@@ -12,3 +12,5 @@ extension P {
     public subscript(index: Int) -> String { fatalError() }
     public func makeIterator() -> AnyIterator<String> { fatalError() }
 }
+
+let x: AnyIterator<String>.Type = S.Iterator.self