pgsql: Add TupleDescGetDefault()

Add TupleDescGetDefault()

This unifies some repetitive code.

Note: I didn't push the "not found" error message into the new
function, even though all existing callers would be able to make use
of it. Using the existing error handling as-is would probably require
exposing the Relation type via tupdesc.h, which doesn't seem
desirable. (Or even if we changed it to just report the OID, it would
inject the concept of a relation containing the tuple descriptor into
tupdesc.h, which might be a layering violation. Perhaps some further
improvements could be considered here separately.)

Discussion: https://www.postgresql.org/message-id/flat/52a125e4-ff9a-95f5-9f61-b87cf447e4da%40eisentraut.org

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ebf76f2753a91615d45f113f1535a8443fa8d076

Modified Files
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src/backend/access/common/tupdesc.c | 25 +++++++++++++++++++++++++
src/backend/commands/tablecmds.c | 17 ++---------------
src/backend/parser/parse_utilcmd.c | 13 ++-----------
src/backend/rewrite/rewriteHandler.c | 16 +---------------
src/include/access/tupdesc.h | 2 ++
5 files changed, 32 insertions(+), 41 deletions(-)