pgsql: Add macro RelationIsPermanent() to report relation permanence

Add macro RelationIsPermanent() to report relation permanence

Previously, to check relation permanence, the Relation's Form_pg_class
structure member relpersistence was compared to the value
RELPERSISTENCE_PERMANENT ("p"). This commit adds the macro
RelationIsPermanent() and is used in appropirate places to simplify the
code. This matches other RelationIs* macros.

This macro will be used in more places in future cluster file encryption
patches.

Discussion: https://postgr.es/m/20210318153134.GH20766@tamriel.snowman.net

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/95d77149c53545a74e0c84717cf8f925b8f6d632

Modified Files
--------------
src/backend/access/gist/gistutil.c | 2 +-
src/backend/access/heap/heapam_handler.c | 2 +-
src/backend/catalog/pg_publication.c | 2 +-
src/backend/commands/tablecmds.c | 10 +++++-----
src/backend/optimizer/util/plancat.c | 3 +--
src/backend/utils/cache/relcache.c | 2 +-
src/include/utils/rel.h | 10 ++++++++--
src/include/utils/snapmgr.h | 3 +--
8 files changed, 19 insertions(+), 15 deletions(-)