[Casting] Make more casts look inside __SwiftValue

The following sequence of casts would previously succeed
```
struct S {}
let s = S() as AnyObject
s as? NSObject
```
The final cast here should fail, since `S` clearly is not a
subclass of NSObject.  But it would previously succeed because
the `as AnyObject` would package the struct into an ObjC-compatible
`__SwiftValue` class.  This latter is an NSObject subclass.

This bug was fixed in the main `swift_dynamicCast` runtime function
some time ago, but not in the `swift_dynamicCastObjCClass` which
is chosen by IRGen to optimize casts to ObjC class types.
This PR changes the latter to test for `__SwiftValue` and fall
back to the former in that case in order to get the correct
handling.  Falling back to `swift_dynamicCast` also ensures that
the contents of the `__SwiftValue` are correctly unwrapped/bridged/etc
as necessary to fully support Swift casting semantics.

Resolves: rdar://111422725

TODO: I've left an XFAILed test here about the behavior of `type(of:)`
with `__SwiftValue` boxes.

Author: tbkka

stdlib/public/runtime/SwiftObject.mm

@@ -1093,6 +1093,22 @@ static bool isObjCForUnownedReference(void *value) {
   if (object == nullptr)
     return nullptr;
 
+  if ([id_const_cast(object) isKindOfClass:[__SwiftValue class]]) {
+    // Source is a `__SwiftValue` container
+    // Unwrap, then use the most general casting machine to do the heavy lifting
+    auto typeValue = getValueFromSwiftValue(reinterpret_cast<__SwiftValue *>(object));
+    const void *result = nullptr;
+    if (swift_dynamicCast(reinterpret_cast<OpaqueValue *>(&result),
+			  const_cast<OpaqueValue *>(typeValue.second),
+			  typeValue.first,
+			  targetType,
+			  DynamicCastFlags::TakeOnSuccess)) {
+      return result;
+    } else {
+      return nullptr;
+    }
+  }
+
   if ([id_const_cast(object) isKindOfClass:class_const_cast(targetType)]) {
     return object;
   }

test/Casting/Casts.swift

@@ -1064,4 +1064,19 @@ CastsTests.test("Don't put AnyHashable inside AnyObject") {
   expectTrue(a === d)
 }
 
+CastsTests.test("__SwiftValue should not be obvious to `is`") {
+  struct S {}
+  let s = S() as AnyObject
+  expectFalse(s is NSObject)
+}
+
+CastsTests.test("type(of:) should look through __SwiftValue")
+.xfail(.always("Known to be broken"))
+.code {
+  struct S {}
+  let s = S() as AnyObject
+  let t = "\(type(of: s))"
+  expectEqual(t, "S")  // Fails: currently says `__SwiftValue`
+}
+
 runAllTests()