Add hooks that enable NSSet, NSDictionary and NSString formatting to apply to...

Add hooks that enable NSSet, NSDictionary and NSString formatting to apply to other types beyond the well-known ones

This is meant to support languages that can do some sort of bridging from<-->to these ObjC types via types that statically vend themselves as Cocoa types, but dynamically have an implementation that does not match any of our well-known types, but where an introspecting formatter can be vended by the bridged language

llvm-svn: 249185