Add a test to validate that we infer the right level of optionality for weak vars.

Author: rudkx

test/Constraints/weak.swift

@@ -0,0 +1,10 @@
+// RUN: %target-typecheck-verify-swift
+
+class C {}
+func overload(x: Int) -> C {}
+func overload(x: Float) -> C? {}
+
+// Ensure that we infer both weak vars as having a single level of
+// optionality. We will emit diagnostics if that is not the case.
+weak var v = overload(x: 1)
+weak var w = overload(x: 1.0)