[libcxx] [test] Avoid spurious test breakage in clang-cl-dll configs with newer CMake

[libcxx] [test] Avoid spurious test breakage in clang-cl-dll configs with newer CMake

The pointer.volatile.pass.cpp test was already marked as XFAIL for
mingw-dll (for reasons explained in the comment above it).

The same issue also appears in clang-cl-dll when built with newer
CMake versions. (It didn't appear with older versions of CMake, as
CMake built the library with the clang-cl flag `-std:c++latest` when
we've requested C++ 20 - which practically built it in c++2b mode with
current clang versions. With current versions of CMake, it passes
`-std:c++20` instead.)

As it succeeds/fails dependent on factors we don't
directly control, mark it as UNSUPPORTED instead of XFAIL.

Differential Revision: https://reviews.llvm.org/D122718