Change copyObject() to use typeof_unqual

Change copyObject() to use typeof_unqual

Currently, when the argument of copyObject() is const-qualified, the
return type is also, because the use of typeof carries over all the
qualifiers.  This is incorrect, since the point of copyObject() is to
make a copy to mutate.  But apparently no code ran into it.

The new implementation uses typeof_unqual, which drops the qualifiers,
making this work correctly.

typeof_unqual is standardized in C23, but all recent versions of all
the usual compilers support it even in non-C23 mode, at least as
__typeof_unqual__.  We add a configure/meson test for typeof_unqual
and __typeof_unqual__ and use it if it's available, else we use the
existing fallback of just returning void *.

This is the second attempt, after the first attempt in commit
4cfce4e62c8 was reverted.  The following two points address problems
with the earlier version:

We test the underscore variant first so that there is a higher chance
that clang used for bitcode also supports it, since we don't test that
separately.

Unlike the typeof test, the typeof_unqual test also tests with a void
pointer similar to how copyObject() would use it, because that is not
handled by MSVC, so we want the test to fail there.

Reviewed-by: David Geier <geidav.pg@gmail.com>
Reviewed-by: Jelte Fennema-Nio <postgres@jeltef.nl>
Discussion: https://www.postgresql.org/message-id/flat/92f9750f-c7f6-42d8-9a4a-85a3cbe808f3%40eisentraut.org