Change plpgsql to depend on main parser's type-declaration grammar,

Change plpgsql to depend on main parser's type-declaration grammar,
rather than having its own somewhat half-baked notion of what a type
declaration looks like.  This is necessary now to ensure that plpgsql
will think a 'timestamp' variable has the same semantics as 'timestamp'
does in the main SQL grammar; and it should avoid divergences in future.