Split out innards of pg_tablespace_location()

Split out innards of pg_tablespace_location()

This creates a src/backend/catalog/pg_tablespace.c supporting file
containing a new function get_tablespace_location(), which lets the code
underlying pg_tablespace_location() be reused for other purposes.

Author: Manni Wood <manni.wood@enterprisedb.com>
Author: Nishant Sharma <nishant.sharma@enterprisedb.com>
Reviewed-by: Vaibhav Dalvi <vaibhav.dalvi@enterprisedb.com>
Reviewed-by: Ian Lawrence Barwick <barwick@gmail.com>
Reviewed-by: Jim Jones <jim.jones@uni-muenster.de>
Reviewed-by: Álvaro Herrera <alvherre@kurilemu.de>
Discussion: https://postgr.es/m/CAKWEB6rmnmGKUA87Zmq-s=b3Scsnj02C0kObQjnbL2ajfPWGEw@mail.gmail.com