Add regression test for fixed crasher

Author: slavapestov

test/Generics/rdar119866847.swift

@@ -0,0 +1,16 @@
+// RUN: %target-typecheck-verify-swift
+
+protocol P1<A> {
+	associatedtype A
+}
+
+protocol P2<A, B>: P1 {
+	associatedtype B
+}
+
+protocol P3s {
+	associatedtype A
+	associatedtype B
+
+	typealias SelfP2 = P2<B, B>
+}