Make plpgsql provide the typmods for its variables to the main parser.

Make plpgsql provide the typmods for its variables to the main parser.

Historically we didn't do this, even though we had the information, because
plpgsql passed its Params via SPI APIs that only include type OIDs not
typmods.  Now that plpgsql uses parser callbacks to create Params, it's
easy to insert the right typmod.  This should generally result in lower
surprise factors, because a plpgsql variable that is declared with a typmod
will now work more like a table column with the same typmod.  In particular
it's the "right" way to fix bug #6020, in which plpgsql's attempt to return
an anonymous record type is defeated by stricter record-type matching
checks that were added in 9.0.  However, it's not impossible that this
could result in subtle behavioral changes that could break somebody's
existing plpgsql code, so I'm afraid to back-patch this change into
released branches.  In those branches we'll have to lobotomize the
record-type checks instead.