Sema, SILGen, ClangImporter: Add special support for Set<T>

Add the following functionality to the Swift compiler:

* covariant subtyping of Set
* upcasting, downcasting of Set
* automatic bridging between Set and NSSet, including
    * NSSet params/return values in ObjC are imported as Set<NSObject>
    * Set params/return values in Swift are visible to ObjC as NSSet

<rdar://problem/18853078> Implement Set<T> up and downcasting

Swift SVN r23751

test/SILGen/Inputs/usr/include/BridgeTestFoundation.h

@@ -16,6 +16,9 @@
 @interface NSDictionary : NSObject
 @end
 
+@interface NSSet : NSObject
+@end
+
 @interface NSNumber : NSObject
 @end