swift234

[Clang importer] Use Clang swift_bridge attribute for bridging.

When an Objective-C class type is annotated with the swift_bridge
value type, bridge it to the named type. Use API notes on Foundation
rather than special cases.

For Objective-C classes where bridging was baked into the Clang
importer (NSString, NSArray, NSDictionary, NSSet), add API notes to
put the appropriate swift_bridge attribute on these Objective-C
classes.

Note: requires Clang update.