patch 8.2.1685: Vim9: cannot declare a constant value

Problem:    Vim9: cannot declare a constant value.
Solution:   Introduce ":const!".
diff --git a/src/eval.c b/src/eval.c
index 18f3a5b..79c5488 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1200,7 +1200,7 @@
     char_u	*endp,
     typval_T	*rettv,
     int		copy,
-    int		flags,    // LET_IS_CONST and/or LET_NO_COMMAND
+    int		flags,    // LET_IS_CONST, LET_FORCEIT, LET_NO_COMMAND
     char_u	*op)
 {
     int		cc;