swift234

Use Optional<T> as the return type Dictionary<KeyType, T> subscript.

This follows the model of objectForKey: from Cocoa, where a sentinel
is returned to indicate no object found.  Moreover, this allows
us to use:

  dict[key] = nil

to zap entries in the dictionary.

As part of this, "hide" the method 'deleteKey' using '_' prefix.

Implements: <rdar://problem/16335854>

Swift SVN r17908