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/testdir/test_cmdline.vim b/src/testdir/test_cmdline.vim
index 3f73d3d..7a4979e 100644
--- a/src/testdir/test_cmdline.vim
+++ b/src/testdir/test_cmdline.vim
@@ -184,6 +184,7 @@
   endif
   for cmd in [
 	\ 'let a = ',
+	\ 'const a = ',
 	\ 'if',
 	\ 'elseif',
 	\ 'while',