When the input is select timestamp '2019-10-29 16:14:42', it's parsed to something like
> 'select timestampliteral()'
`timestampliteral` is a valid function name in TiDB, used to implement the timestamp literal.
However, if the input is 'select timestampliteral()', we should reject this query.
This PR renames `timestampliteral` to avoid conflict.