[libc++] Mark convert_copy.pass.cpp as UNSUPPORTED on clang-13 (i.e. trunk).

Because the constexpr-time codepath triggers a Clang bug. It seems
that Clang compiles it okay in release mode, but when Clang itself
is compiled in debug mode (with assertions turned on), this input
triggers an assertion failure in Clang itself. See comments on D96385
and Clang bug report https://bugs.llvm.org/show_bug.cgi?id=45879

This commit should get the debug-mode buildbots back to green.