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

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.