Add a test for referencing a type nested in a generic class from a derived class

This was reported in rdar://problem/31529413 and appears to have already
been fixed. Adding this test to ensure we have some coverage for this.

Author: rudkx

test/Generics/nested_type_reference.swift

@@ -0,0 +1,15 @@
+// RUN: %target-typecheck-verify-swift
+
+enum E { case A }
+
+class C<T> {
+  struct Nested {
+    var value: T
+  }
+}
+
+class D: C<E> {
+  func test() {
+    _ = Nested(value: .A)
+  }
+}