Support domains over composite types in PL/Perl.
In passing, don't insist on rsi->expectedDesc being set unless we
actually need it; this allows succeeding in a couple of cases where
PL/Perl functions returning setof composite would have failed before,
and makes the error message more apropos in other cases.
Discussion: https://postgr.es/m/4206.
1499798337@sss.pgh.pa.us