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c078479ce9
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.