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 3e9ca5260, fixing build
breakage from dc7420c2c and back-patching of f10f0ae42.

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

@@ -468,6 +468,7 @@ typedef struct ViewOptions
 #define RelationIsMapped(relation) \
 	((relation)->rd_rel->relfilenode == InvalidOid)
 
+#ifndef FRONTEND
 /*
  * RelationGetSmgr
  *		Returns smgr file handle for a relation, opening it if needed.
@@ -488,6 +489,7 @@ RelationGetSmgr(Relation rel)
 		smgrsetowner(&(rel->rd_smgr), smgropen(rel->rd_node, rel->rd_backend));
 	return rel->rd_smgr;
 }
+#endif							/* !FRONTEND */
 
 /*
  * RelationOpenSmgr