Re: Type assertions without GCC builtins

[Replace __builtin_types_compatible_p with _Generic]

Here is an updated and expanded patch set.

The first patch replaces __builtin_types_compatible_p with _Generic. It
is similar to the previous patches in this thread. I added some more
code comments about what is supposed to work and how.

I did not include any C++ support here. I feel this ought to be done as
a separate patch. This patch already carries portability risk and will
require some buildfarm animal changes, so it seems better not to overlay
any C++ issues.

As we had discussed before, this does not work with VS 2019. We no
longer have CI coverage for VS 2019, and I raised this issue at the CI
session at the pgconf.dev unconference, and the consensus was that we
should drop VS 2019 support. So I'm including a patch for that here.
This is just a documentation patch. But we can also drop some support
for early VS 2022 versions (_MSC_VER < 1933), since it's unlikely that
those would be relevant.

And then, since we are dropping support for gcc 4.8, which means RHEL 7,
we also drop support for some old ICU and Tcl versions. I have included
patches for that there, too.

Required buildfarm updates:

VS 2019:
-
https://buildfarm.postgresql.org/cgi-bin/show_history.pl?nm=drongo&br=master

RHEL 7:
-
https://buildfarm.postgresql.org/cgi-bin/show_history.pl?nm=siskin&br=master
-
https://buildfarm.postgresql.org/cgi-bin/show_history.pl?nm=rhinoceros&br=master
- https://buildfarm.postgresql.org/cgi-bin/show_history.pl?nm=buri&br=master