pgsql: Move code for managing PartitionDescs into a new file, partdesc.

Move code for managing PartitionDescs into a new file, partdesc.c

This is similar in spirit to the existing partbounds.c file in the
same directory, except that there's a lot less code in the new file
created by this commit. Pending work in this area proposes to add a
bunch more code related to PartitionDescs, though, and this will give
us a good place to put it.

Discussion: http://postgr.es/m/CA+TgmoZUwPf_uanjF==gTGBMJrn8uCq52XYvAEorNkLrUdoawg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/1bb5e78218107db543086e71fc162d36a955b08e

Modified Files
--------------
src/backend/catalog/heap.c | 1 +
src/backend/catalog/partition.c | 16 ---
src/backend/catalog/pg_constraint.c | 1 -
src/backend/commands/indexcmds.c | 2 +-
src/backend/commands/tablecmds.c | 1 +
src/backend/commands/trigger.c | 1 +
src/backend/executor/execPartition.c | 1 +
src/backend/optimizer/util/inherit.c | 1 +
src/backend/optimizer/util/plancat.c | 2 +-
src/backend/partitioning/Makefile | 2 +-
src/backend/partitioning/partbounds.c | 6 +-
src/backend/partitioning/partdesc.c | 221 ++++++++++++++++++++++++++++++++++
src/backend/utils/cache/partcache.c | 124 -------------------
src/backend/utils/cache/relcache.c | 57 +--------
src/include/catalog/partition.h | 15 ---
src/include/partitioning/partdesc.h | 39 ++++++
src/include/utils/partcache.h | 1 -
17 files changed, 273 insertions(+), 218 deletions(-)