pgsql: Add a reverse-translation column number array to struct AppendRe

Add a reverse-translation column number array to struct AppendRelInfo.

This provides for cheaper mapping of child columns back to parent
columns.  The one existing use-case in examine_simple_variable()
would hardly justify this by itself; but an upcoming bug fix will
make use of this array in a mainstream code path, and it seems
likely that we'll find other uses for it as we continue to build
out the partitioning infrastructure.

Discussion: https://postgr.es/m/12424.1575168015@sss.pgh.pa.us

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/ce76c0ba53e4bd0daf3db7a703671b27797b7244

Modified Files
--------------
src/backend/nodes/copyfuncs.c             |  2 ++
src/backend/nodes/equalfuncs.c            |  2 ++
src/backend/nodes/nodeFuncs.c             |  1 +
src/backend/nodes/outfuncs.c              |  2 ++
src/backend/optimizer/prep/prepjointree.c | 18 +++++++++++++-----
src/backend/optimizer/util/appendinfo.c   | 26 ++++++++++++++++++++------
src/backend/utils/adt/selfuncs.c          | 24 ++++++------------------
src/include/nodes/pathnodes.h             | 27 ++++++++++++++++-----------
8 files changed, 62 insertions(+), 40 deletions(-)