swift234

Don't allow existentials to be used where we need a witness table.

If we end up trying to form a substitution where the replacement type
is an existential and there is a conformance to a non-@objc protocol
(i.e., a conformance where a witness table is required), complain in
Sema rather than crashing in IRGen. Fixes rdar://problem/21087341, but
the existential/generic interaction is still quite broken.

Swift SVN r29133