swift234

Use the dynamic casting infrastructure for casts that bridge from Objective-C.

A checked cast such as "x as String" or "x as? [String]", where x is of
class or Objective-C existential type, is now handled as a normal
checked cast rather than a Sema-generated call to the corresponding
witness. This eliminates a pile of hairy code from constraint
application and takes a step toward <rdar://problem/17408934>.

The part of the switch_objc.swift test I removed wasn't testing
anything useful; that's what <rdar://problem/17408934> is about.



Swift SVN r20970