SR-3871: Dynamic casting of existentials stored in Obj-C references
Arbitrary Swift objects get packaged into __SwiftValue containers so
that pointers to them can be passed into Obj-C. (Obviously, Obj-C
code can't do anything particularly useful with such pointers other
than refcount them and give them back to Swift code.) Those values come
back into Swift as either `Any` (existential box) or `AnyObject`
(anonymous object pointer) values. Dynamically casting those requires
first inspecting the outer value to get access to the actual type and
value in the __SwiftValue container.
The tryDynamicCastBoxedSwiftValue() function that handles this
was missing a check for the `Any` case, which is why directly
casting from `Any` would routinely fail.
Resolves SR-3871
The real work is done in checkDynamicCastFromOptional. This code tried to unwrap the source and returned the payload on success. It only did this once, so a type like Optional<Optional<Int>> would come out as Optional<Int> and then a cast to Int would fail.
This change makes checkDynamicCastFromOptional recursive, which makes it unwrap as many levels of Optional as it encounters.
Fixes rdar://problem/40171034 and SR-7664.
