Make our back branches build under -fkeep-inline-functions.

Add "#ifndef FRONTEND" where necessary to make pg_waldump build
on compilers that don't elide unused static-inline functions.

This back-patches relevant parts of commit 3e9ca52, fixing build
breakage from dc7420c and back-patching of f10f0ae.

Per recently-resurrected buildfarm member castoroides.  We aren't
expecting castoroides to build anything newer than v11, but we
might as well clean up the intermediate branches while at it.

src/include/utils/rel.h

@@ -538,6 +538,7 @@ typedef struct ViewOptions
 	(RELKIND_HAS_STORAGE((relation)->rd_rel->relkind) && \
 	 ((relation)->rd_rel->relfilenode == InvalidOid))
 
+#ifndef FRONTEND
 /*
  * RelationGetSmgr
  *		Returns smgr file handle for a relation, opening it if needed.
@@ -558,6 +559,7 @@ RelationGetSmgr(Relation rel)
 		smgrsetowner(&(rel->rd_smgr), smgropen(rel->rd_node, rel->rd_backend));
 	return rel->rd_smgr;
 }
+#endif							/* !FRONTEND */
 
 /*
  * RelationCloseSmgr

src/include/utils/snapmgr.h

@@ -97,11 +97,13 @@ extern PGDLLIMPORT SnapshotData CatalogSnapshotData;
 	((snapshot)->snapshot_type == SNAPSHOT_MVCC || \
 	 (snapshot)->snapshot_type == SNAPSHOT_HISTORIC_MVCC)
 
+#ifndef FRONTEND
 static inline bool
 OldSnapshotThresholdActive(void)
 {
 	return old_snapshot_threshold >= 0;
 }
+#endif
 
 extern Snapshot GetTransactionSnapshot(void);
 extern Snapshot GetLatestSnapshot(void);