Patch to allow domains over composite types

typecmds.c says:
"Domains over composite types might be made to work in the future, but
not today."

Attached is a patch that allows domains over composite types, together
with test cases in domaincomp.sql. A domain over a composite type has
typtype TYPTYPE_DOMAIN, but typrelid and typrelkind are empty: that
information is only available in the pg_type record of the base type.
The remainder of the patch follows from that choice. While parsing a
record expression into a row type, an extra coercion node had to be
inserted to ensure that the domain checks are called.

All regression tests are ok, comments are highly appreciated.

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Yeb Havinga
http://www.mgrid.net/
Mastering Medical Data