patch 8.1.2348: :const cannot be followed by "| endif"

Problem:    :const cannot be followed by "| endif".
Solution:   Check following command for :const. (closes #5269)
            Also fix completion after :const.
diff --git a/src/eval.c b/src/eval.c
index ace1e01..b65f29e 100644
--- a/src/eval.c
+++ b/src/eval.c
@@ -1521,7 +1521,7 @@
     int		c;
     char_u	*p;
 
-    if (cmdidx == CMD_let)
+    if (cmdidx == CMD_let || cmdidx == CMD_const)
     {
 	xp->xp_context = EXPAND_USER_VARS;
 	if (vim_strpbrk(arg, (char_u *)"\"'+-*/%.=!?~|&$([<>,#") == NULL)