Make sure artificial subclasses work with '===' and casting (#17765)

Dynamic subclasses aren't /really/ valid Swift type metadata, but
they can still be used as values of type AnyClass. Make sure we
don't assert when that happens.

No intended functionality change.

(cherry picked from commit 2345a54)

Author: jrose-apple

stdlib/public/runtime/Metadata.cpp

@@ -593,7 +593,7 @@ swift::swift_getObjCClassFromMetadata(const Metadata *theMetadata) {
 
   // Otherwise, the input should already be a Swift class object.
   auto theClass = cast<ClassMetadata>(theMetadata);
-  assert(theClass->isTypeMetadata() && !theClass->isArtificialSubclass());
+  assert(theClass->isTypeMetadata());
   return theClass;
 }
 

test/Interpreter/SDK/KVO.swift

@@ -55,3 +55,20 @@ foo.addObserver(foo, forKeyPath: "foo", options: [], context: &kvoContext)
 let bar = foo.foo
 // CHECK-NEXT: 0
 print(bar)
+
+let fooClass: AnyClass = object_getClass(foo)!
+precondition(fooClass !== Foo.self, "no KVO subclass?")
+precondition(fooClass is Foo.Type, "improper KVO subclass")
+precondition(!(fooClass is Observer.Type), "improper KVO subclass")
+
+let fooClassAsObject: AnyObject = fooClass
+precondition(fooClassAsObject !== Foo.self, "no KVO subclass?")
+precondition(fooClassAsObject is Foo.Type, "improper KVO subclass")
+precondition(!(fooClassAsObject is Observer.Type), "improper KVO subclass")
+
+let fooClassAsAny: Any = fooClass
+precondition(fooClassAsAny is Foo.Type, "improper KVO subclass")
+precondition(!(fooClassAsAny is Observer.Type), "improper KVO subclass")
+
+// CHECK-NEXT: class metadata checks okay
+print("class metadata checks okay")