Ensure we build generated headers at the start of some more cases.

"make installcheck" and some related cases, when invoked from the toplevel
directory, start out by doing "make all" in src/test/regress.  Since that's
one make recursion level down, the submake-generated-headers target will
do nothing, causing us to fail to create/update generated headers before
building pg_regress.  This is, I believe, a new failure mode induced by
commit 3b8f6e75, so let's fix it.  To do so, we have to invoke
submake-generated-headers at the top level.

Discussion: https://postgr.es/m/0401efec-68f1-679d-3ea3-21d4e8dd11af@gmail.com