[test] Add AnyObject construction test case

This is a case where a constructor overload choice
might currently have an optional opened type, so
make sure we can handle it. Arguably we shouldn't
even be introducing constructors into the overload
set here though, as it's invalid anyway.

Author: hamishknight

test/Constraints/dynamic_lookup.swift

@@ -453,3 +453,10 @@ func test_dynamic_subscript_accepts_type_name_argument() {
     // expected-note@-2 {{use '.self' to reference the type object}} {{20-20=.self}}
   }
 }
+
+func testAnyObjectConstruction(_ x: AnyObject) {
+  AnyObject() // expected-error {{protocol type 'AnyObject' cannot be instantiated}}
+
+  // FIXME(SR-15210): This should also be rejected.
+  _ = type(of: x).init()
+}