Clean up parsing of CREATE TRIGGER's argument list.

Use ColLabel in place of ColId, so that reserved words are accepted as if
they were not reserved.  Also, remove BCONST and XCONST, which were never
documented as allowed.  Allowing those exposes to users an implementation
detail, namely the format in which the lexer outputs such constants, that
seems unwise to expose.

No documentation change needed, since this just makes the code act more
like you'd expect from reading the CREATE TRIGGER man page.

Per complaint from Szymon Guz and subsequent discussion.