[SourceKit] Make sourcektid_request_handle_t const void*

Author: ahoppen

tools/SourceKit/tools/sourcekitd/include/sourcekitd/sourcekitd.h

@@ -590,7 +590,7 @@ sourcekitd_response_t
 sourcekitd_send_request_sync(sourcekitd_object_t req);
 
 /// Used to cancel a request that has been invoked asynchronously.
-typedef void *sourcekitd_request_handle_t;
+typedef const void *sourcekitd_request_handle_t;
 
 #if SOURCEKITD_HAS_BLOCKS
 /// Receives the response of an asynchronous request or notification.