headerscheck and cpluspluscheck should skip the recently-added
cmdtaglist.h header, since (like kwlist.h and some other similarly-
designed headers) it's not meant to be included standalone.
evtcache.h was missing an #include to support its usage of Bitmapset.
typecmds.h was missing an #include to support its usage of ParseState.
The first two of these were evidently oversights in commit 2f9661311.
I didn't track down exactly which change broke typecmds.h, but it
must have been some rearrangement in one of its existing inclusions,
because it's referenced ParseState for quite a long time and there
were not complaints from these checking programs before.
src/include/common/unicode_combining_table.h is currently not meant to
be included standalone. Things could be refactored to allow it, but
that would be beyond the present purpose. So adding an exclusion here
seems best.
Discussion: https://www.postgresql.org/message-id/10754.1579535012@sss.pgh.pa.us
We already had "cpluspluscheck", which served the dual purposes of
verifying that headers compile standalone and that they compile as C++.
However, C++ compilers don't have the exact same set of error conditions
as C compilers, so this doesn't really prove that a header will compile
standalone as C.
Hence, add a second script that's largely similar but runs the C
compiler not C++.
Also add a bit more documentation than the none-at-all we had before.
Discussion: https://postgr.es/m/14803.1566175851@sss.pgh.pa.us
