swift234

Add stub implementations of the dictionary upcast entry points.

This introduces _dictionaryUpCast and _dictionaryBridgeToObjectiveC,
which will be used for upcasting for non-bridged and bridged key/value
types, respectively. _dictionaryUpCast is a horrible copying O(n)
implementation where we should be able to provide an O(1)
implementation, and _dictionaryBridgeToObjectiveC is similarly
awful. Hence, this is not for <rdar://problem/16852016>, but is merely
a stub to let us make progress on upcasting in the frontend
(<rdar://problem/17289296>).

Swift SVN r18867