[tests] Adding regression tests for SR-12946

Author: LucianoPAlmeida

test/expr/cast/metatype_casts.swift

@@ -49,3 +49,10 @@ use(C.self as P.Type) // expected-error{{cannot convert value of type 'C.Type' t
 
 use(E.self as P.Protocol) // expected-error{{cannot convert value of type 'E.Type' to type 'P.Protocol' in coercion}}
 use(E.self as P.Type)
+
+// SR-12946
+func SR12946<T>(_ e: T) {
+  _ = AnyObject.self is T.Type // OK
+}
+
+SR12946(1 as AnyObject)