[unsafe-to-concat] Mark LigatureSubst

Failures down to two:

209/401 harfbuzz:shaping+aots / gpos6                            FAIL             0.06s   exit status 1
265/401 harfbuzz:shaping+aots / lookupflag_ignore_attach         FAIL             0.06s   exit status 1

src/hb-ot-layout-gsub-table.hh

@@ -836,7 +836,10 @@ struct Ligature
 			      &match_end,
 			      match_positions,
 			      &total_component_count)))
+    {
+      c->buffer->unsafe_to_concat (c->buffer->idx, match_end);
       return_trace (false);
+    }
 
     ligate_input (c,
 		  count,