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

lib/Sema/Constraint.cpp

@@ -372,6 +372,8 @@ StringRef swift::constraints::getName(ConversionRestrictionKind kind) {
     return "[array-upcast]";
   case ConversionRestrictionKind::DictionaryUpcast:
     return "[dictionary-upcast]";
+  case ConversionRestrictionKind::SetUpcast:
+    return "[set-upcast]";
   case ConversionRestrictionKind::BridgeToObjC:
     return "[bridge-to-objc]";
   case ConversionRestrictionKind::BridgeFromObjC: