Add a test case that exercises opaque existential box's value witness assignWith

rdar://31955457

Author: aschwaighofer

test/Interpreter/dictOfAny.swift

@@ -0,0 +1,12 @@
+// RUN: %target-run-simple-swift | %FileCheck %s
+// REQUIRES: executable_test
+// REQUIRES: objc_interop
+
+import Foundation
+
+var dict: [String: Any] = ["foo": 0x11111111 as NSNumber]
+print(dict)  // succeeds
+// CHECK: ["foo": 286331153]
+dict["foo"] = 0x22222222
+print(dict)  // used to crash
+// CHECK: ["foo": 572662306]