backend *.c #include cleanup (IWYU)

I played with include-what-you-use (IWYU), "a tool for use with clang to
analyze #includes in C and C++ source files".[0] I came across this via
clangd (the language server), because clangd (via the editor) kept
suggesting a bunch of #includes to remove. And I suppose it was right.

So as a test, I ran IWYU over the backend *.c files and removed all the
#includes it suggested. (Note that IWYU also suggests to *add* a bunch
of #includes, in fact that is its main purpose; I didn't do this here.)
In some cases, a more specific #include replaces another less specific
one. (To keep the patch from exploding in size, I ignored for now all
the suggestions to replace catalog/pg_somecatalog.h with
catalog/pg_somecatalog_d.h.) This ended up with the attached patch,
which has

432 files changed, 233 insertions(+), 1023 deletions(-)

I tested this with various compilation options (assert, WAL_DEBUG,
LOCK_DEBUG, different geqo variants, etc.) to make sure a header wasn't
just used for some conditional code section. Also, again, this patch
touches just *.c files, so nothing declared from header files changes in
hidden ways.

Also, as a small example, in src/backend/access/transam/rmgr.c you'll
see some IWYU pragma annotations to handle a special case there.

The purpose of this patch is twofold: One, it's of course a nice
cleanup. Two, this is a test how well IWYU might work for us. If we
find either by human interpretation that a change doesn't make sense, or
something breaks on some platform, then that would be useful feedback
(perhaps to addressed by more pragma annotations or more test coverage).

(Interestingly, IWYU has been mentioned in src/tools/pginclude/README
since 2012. Has anyone else played with it? Was it not mature enough
back then?)

[0]: https://include-what-you-use.org/