IRGen: Get objc protocol erasure working.

Don't try to emit witness tables for protocols that don't need them when emitting a protocol erasure. (We'll need to eventually ensure that ObjC method metadata gets generated for the conforming methods, but we don't yet, so this will only work for types that already have ObjC-dispatchable methods.)

Swift SVN r5610

test/IRGen/Inputs/usr/include/Gizmo.h

@@ -46,3 +46,16 @@ typedef long NSInteger;
 struct NSRect NSMakeRect(double, double, double, double);
 struct NSRect NSInsetRect(struct NSRect, double, double);
 NSString *NSStringFromRect(struct NSRect r);
+
+@protocol NSRuncing
+- (void)runce;
+@end
+
+@protocol NSFunging
+- (void)funge;
+@end
+
+@interface NSSpoon <NSRuncing, NSFunging>
+- (void)runce;
+- (void)funge;
+@end