[subset] Use a std::move on set_t when feasible

src/hb-ot-layout-gsubgpos.hh

@@ -1361,7 +1361,7 @@ static void context_closure_recurse_lookups (hb_closure_context_t *c,
 
     covered_seq_indicies->add (seqIndex);
     if (has_pos_glyphs) {
-      c->push_cur_active_glyphs () = pos_glyphs;
+      c->push_cur_active_glyphs () = std::move (pos_glyphs);
     } else {
       c->push_cur_active_glyphs ().set (*c->glyphs);
     }