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

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/877a024902a73732d9f976804aee9699dcbe1d90

Modified Files
--------------
src/backend/catalog/Makefile | 1 +
src/backend/catalog/meson.build | 1 +
src/backend/catalog/pg_tablespace.c | 90 +++++++++++++++++++++++++++++++++++++
src/backend/utils/adt/misc.c | 62 ++-----------------------
src/include/catalog/pg_tablespace.h | 2 +
5 files changed, 98 insertions(+), 58 deletions(-)