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/