swift234

Introduce _ConditionallyBridgedToObjectiveC.bridgeFromObjectiveCConditional.

This entry point is used in conditional downcasts (as?) to attempt to
bridge from an Objective-C class down to a specific native type (e.g.,
array, dictionary), bridging all elements eagerly so that it can
produce nil if the bridging would fail.

This is the scaffolding for <rdar://problem/17319154>, and makes the
example there work, but there is much more cleanup and optimization to
do.



Swift SVN r18999