Don't rely on system NS_SWIFT_SENDABLE

Author: DougGregor

test/IRGen/Inputs/objc_protocol_sendable.h

@@ -1,5 +1,5 @@
 #import <Foundation/Foundation.h>
 
 @interface A: NSObject
-- (id <NSObject>)foo NS_SWIFT_SENDABLE;
+- (id <NSObject>)foo __attribute__((swift_attr("@Sendable")));
 @end