Added typing for the built-in generic functions that are available from the
:data:`_sql.func` namespace, which accept a particular set of arguments and
return a particular type, such as for :class:`_sql.count`,
:class:`_sql.current_timestamp`, etc.
Fixes: #9129
Change-Id: I1a2e0dcca3048c77e84dc786843a7df05c457dfa
Mike Bayer