[myanmar] Use buffer->sort() to sort cluster

This can possibly produce more granular clusters.

src/hb-ot-shape-complex-myanmar.cc

@@ -391,9 +391,8 @@ initial_reordering_consonant_syllable (hb_buffer_t *buffer,
     }
   }
 
-  buffer->merge_clusters (start, end);
   /* Sit tight, rock 'n roll! */
-  hb_stable_sort (info + start, end - start, compare_myanmar_order);
+  buffer->sort (start, end, compare_myanmar_order);
 }
 
 static void