Merge catalog/pg_foo_fn.h headers back into pg_foo.h headers.

Merge catalog/pg_foo_fn.h headers back into pg_foo.h headers.

Traditionally, include/catalog/pg_foo.h contains extern declarations
for functions in backend/catalog/pg_foo.c, in addition to its function
as the authoritative definition of the pg_foo catalog's rowtype.
In some cases, we'd been forced to split out those extern declarations
into separate pg_foo_fn.h headers so that the catalog definitions
could be #include'd by frontend code.  That problem is gone as of
commit 9c0a0de4c, so let's undo the splits to make things less
confusing.

Discussion: https://postgr.es/m/23690.1523031777@sss.pgh.pa.us