Improve plpgsql's handling of record field references by forcing all potential

Improve plpgsql's handling of record field references by forcing all potential
field references in SQL expressions to have RECFIELD datum-array entries at
parse time.  If it turns out that the reference is actually to a SQL column,
the RECFIELD entry is useless, but it costs little.  This allows us to get rid
of the previous use of FieldSelect applied to a whole-row Param for the record
variable; which was not only slower than a direct RECFIELD reference, but
failed for references to system columns of a trigger's NEW or OLD record.
Per report and fix suggestion from Dean Rasheed.