stdcountof-h: Work around a clang bug. Reported by Basil L. Contovounesios <basil@contovou.net> in <https://lists.gnu.org/archive/html/bug-gnulib/2025-06/msg00073.html>. * lib/stdcountof.in.h (_gl_verify_is_array): Disable the checking on clang. * tests/test-stdcountof-h.c (test_func): Add another test case.