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/cmdexpand.c b/src/cmdexpand.c
index 7381b0e..1a7f9f2 100644
--- a/src/cmdexpand.c
+++ b/src/cmdexpand.c
@@ -1508,6 +1508,7 @@
 	    break;
 #endif
 #ifdef FEAT_EVAL
+	case CMD_const:
 	case CMD_let:
 	case CMD_if:
 	case CMD_elseif: