[TypeChecker] Add test-case for rdar://problem/39209245

Author: xedin

test/Constraints/rdar39209245.swift

@@ -0,0 +1,15 @@
+// RUN: %target-typecheck-verify-swift
+
+struct S: Hashable {
+  let e: E?
+}
+
+enum E: Hashable {
+  case foo
+  case bar
+}
+
+let a = S(e: .foo)
+let b = S(e: .bar)
+
+_ = a == b