pgsql: Add index_get_partition convenience function

Add index_get_partition convenience function

This new function simplifies some existing coding, as well as supports
future patches.

Discussion: https://postgr.es/m/201901222145.t6wws6t6vrcu@alvherre.pgsql
Reviewed-by: Amit Langote, Jesper Pedersen

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/a6da0047158b8a227f883aeed19eb7fcfbef11fb

Modified Files
--------------
src/backend/catalog/partition.c  | 36 ++++++++++++++++++++++++++++++++++++
src/backend/commands/tablecmds.c | 40 +++++++++++-----------------------------
src/include/catalog/partition.h  |  1 +
3 files changed, 48 insertions(+), 29 deletions(-)