Fill in extended existential support for type(of:) and mirror
type(of:) now returns the dynamic type of the contents of
an extended existential (just like it does for a regular existential)
Mirror can now reflect fields of a value stored inside an extended
existential (just like it can with a regular existential). This
requires type(of:) support, since Mirror internals use that to
determine how to reflect a value.
Resolves rdar://102906195
