Add regression test for rdar://48994748

Author: slavapestov

validation-test/compiler_crashers_2_fixed/0200-rdar48994748.swift

@@ -0,0 +1,15 @@
+// RUN: not %target-swift-frontend -typecheck %s
+
+protocol P {
+  associatedtype T
+}
+
+class G<T : P> {
+  init(_: T.T) {}
+}
+
+class Sub : G<S> {}
+
+struct S : P {}
+
+Sub()