pgsql: Don't bother to attach column name lists to RowExprs of named ty

Don't bother to attach column name lists to RowExprs of named types.

If a RowExpr is marked as returning a named composite type, we aren't
going to consult its colnames list; we'll use the attribute names
shown for the type in pg_attribute.  Hence, skip storing that list,
to save a few nanoseconds when copying the expression tree around.

Discussion: https://postgr.es/m/2950001.1638729947@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d7b5c071dd6af2b81a7042dc60295061c7230cdc

Modified Files
--------------
src/backend/optimizer/prep/prepjointree.c |  6 +++---
src/backend/optimizer/util/var.c          |  1 +
src/backend/rewrite/rewriteManip.c        |  6 +++---
src/include/nodes/primnodes.h             | 16 +++++++---------
4 files changed, 14 insertions(+), 15 deletions(-)