vim-patch:8.2.1471: :const only locks the variable, not the value (#12765)

Problem:    :const only locks the variable, not the value.
Solution:   Lock the value as ":lockvar 1 var" would do. (closes vim/vim#6719)
241572794f

src/nvim/eval.c

@@ -9045,7 +9045,7 @@ static void set_var_const(const char *name, const size_t name_len,
   }
 
   if (is_const) {
-    v->di_tv.v_lock |= VAR_LOCKED;
+    tv_item_lock(&v->di_tv, 1, true);
   }
 }