Replace insert by emplace and UNREACHABLE by CHECK_RETURN

Using emplace avoids a call to make_pair, and CHECK_RETURN is more
adapted to the situation were we check the return value of insert/emplace.

Author: romainbrenguier

src/goto-symex/symex_start_thread.cpp

@@ -68,10 +68,9 @@ void goto_symext::symex_start_thread(statet &state)
     lhs.set_level_0(t);
 
     // set up L1 name
-    if(!state.level1.current_names.insert(
-        std::make_pair(lhs.get_l1_object_identifier(),
-                       std::make_pair(lhs, 0))).second)
-      UNREACHABLE;
+    auto emplace_result = state.level1.current_names.emplace(
+      lhs.get_l1_object_identifier(), std::make_pair(lhs, 0));
+    CHECK_RETURN(emplace_result.second);
     state.rename(lhs, ns, goto_symex_statet::L1);
     const irep_idt l1_name=lhs.get_l1_object_identifier();
     // store it