CURRENT_DATE/CURRENT_TIME/CURRENT_TIMESTAMP can be used as SQL keywords
(without parentheses) and therefore there is a special rule in the
grammar to accommodate this.
Previously, this rule was also catching the parenthesised version of those functions too,
not allowing the {fn <functionName>()} to be used. E.g.:
{fn current_time(2)} or {fn current_timestamp()}
Now, the grammar rule catches only the keyword versions and all the parenthesised
versions go through the normal function resolution. As a consequence the validation
of the precision is moved from the parser lever (ExpressionBuilder) to the function
implementations.
Fixes: #41240
(cherry picked from commit bfbc9f140144b5a35aa29008b58bf58074419853)
3fd081528d