pgsql: Un-break whole-row Vars referencing domain-over-composite types.

Un-break whole-row Vars referencing domain-over-composite types.

In commit ec62cb0aa, I foolishly replaced ExecEvalWholeRowVar's
lookup_rowtype_tupdesc_domain call with just lookup_rowtype_tupdesc,
because I didn't see how a domain could be involved there, and
there were no regression test cases to jog my memory.  But the
existing code was correct, so revert that change and add a test
case showing why it's necessary.  (Note: per comment in struct
DatumTupleFields, it is correct to produce an output tuple that's
labeled with the base composite type, not the domain; hence just
blindly looking through the domain is correct here.)

Per bug #17515 from Dan Kubb.  Back-patch to v11 where domains over
composites became a thing.

Discussion: https://postgr.es/m/17515-a24737438363aca0@postgresql.org

Branch
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REL_11_STABLE

Details
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https://git.postgresql.org/pg/commitdiff/199aac8b2f74dcc19cf88ed7072617d753d9506c

Modified Files
--------------
src/backend/executor/execExprInterp.c |  6 +++++-
src/test/regress/expected/domain.out  | 23 +++++++++++++++++++++++
src/test/regress/sql/domain.sql       |  9 +++++++++
3 files changed, 37 insertions(+), 1 deletion(-)