swift234

Introduce the ObjCSelector class to store an Objective-C selector.

We have to work with selectors quite often, so provide an efficient
representation for them. Switch ObjCAttr over to this representation,
which has the nice property that it efficiently represents implicit
@objc attributes with names and allows us to overwrite the Objective-C
name without losing all source information. Addresses
<rdar://problem/16478678>, and sets us up for dealing with selectors
better.

Swift SVN r16327