* SR-14635: Casts to NSCopying should not always succeed
The runtime dynamic casting logic explores a variety of strategies for
each cast request. One of the last options is to wrap the source
in a `__SwiftValue` box so it can bridge to Obj-C. The previous
code was overly aggressive about such boxing; it performed the boxing
for any source type and only checked to verify that the `__SwiftValue`
box itself was compatible with the destination.
Among other oddities, this results in the behavior discussed
in SR-14635, where any Swift or Obj-C type will always successfully cast
to NSCopying because `__SwiftValue` is compatible with NSCopying.
This is actually two subtly different issues:
* Class types should not be subject to `__SwiftValue` boxing at all.
Casting class types to class existentials is already handled elsewhere,
so this function should just reject any source with class type.
* Non-class types should be boxed only when being assigned to
an AnyObject (an "unconstrained class existential"). If
the class existential has constraints, it is by definition
a class-constrained existential which should not receive
any non-class object.
To solve these, this PR disables `__SwiftValue` boxing in two cases:
1. If the source is a class (reference) type.
2. If the destination has constraints
Resolves SR-14635
Resolves rdar://78224322
* Avoid boxing class metatypes on Darwin
But continue boxing
* Non-class metatypes on all platforms
* All metatypes on non-Darwin platforms
Obj-C interop requires that we do not box class metatypes;
those must be usable as simple pointers when passed to Obj-C.
But no other metatype is object-compatible, so we have to
continue boxing everything else.
* Split out ObjC-specific test cases
