pgsql: Improve plpgsql's handling of record field references by forcing

Log Message:
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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.

Modified Files:
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    pgsql/src/pl/plpgsql/src:
        gram.y (r1.137 -> r1.138)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/gram.y?r1=1.137&r2=1.138)
        pl_comp.c (r1.147 -> r1.148)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_comp.c?r1=1.147&r2=1.148)
        pl_scanner.c (r1.3 -> r1.4)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/pl_scanner.c?r1=1.3&r2=1.4)
        plpgsql.h (r1.127 -> r1.128)
        (http://anoncvs.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/plpgsql.h?r1=1.127&r2=1.128)