Verify that nil can cast to AnyHashable (swiftlang#34434)

Optional conditionally conforms to Hashable, so for
example `Optional<Int>` must cast to AnyHashable
_even if it contains `nil`_.  This was broken up
through Swift 5.3 and was fixed by the new dynamic
cast runtime.

Add a test to ensure this stays fixed.

Author: tbkka

test/Casting/Casts.swift

@@ -761,4 +761,12 @@ CastsTests.test("Async function types") {
   expectFalse(asyncFnType is (() -> Void).Type)
 }
 
+// `Optional<Int>` is Hashable, so it must cast to AnyHashable,
+// even if it contains a nil.  (This was broken in 5.3 and earlier,
+// but was fixed by the new dynamic cast runtime.)
+CastsTests.test("Optional nil -> AnyHashable") {
+  let a : Int? = nil
+  expectNotNil(a as? AnyHashable)
+}
+
 runAllTests()