swift234

Recognize the objc_bridge and objc_bridge_mutable
attributes and create implicit conversions for them.

Also, when generating Clang modules for import, set the
appropriate macro to make CoreFoundation actually apply
those attributes to its typedefs.

Also, give shared linkage to imported class method
definitions.

The net effect is that CFString now automatically
converts to NSString and vice-versa.

Depends on Clang r208756 in order to preserve attributes
applied to tag types in multiple typedef declarations.

Swift SVN r18069