[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

libcxx/test/std/input.output/iostream.format/output.streams/ostream.formatted/ostream.inserters.arithmetic/pointer.volatile.pass.cpp

@@ -10,12 +10,22 @@
 
 // UNSUPPORTED: c++03, c++11, c++14, c++17, c++20
 
-// This test fails on MinGW DLL configurations, due to
+// This test fails in Windows DLL configurations, due to
 // __exclude_from_explicit_instantiation__ not behaving as it should in
 // combination with dllimport (https://llvm.org/PR41018), in combination
 // with running tests in c++2b mode while building the library in c++20 mode.
-// (If the library was built in c++2b mode, this test would succeed.)
-// XFAIL: target={{.+}}-windows-gnu && windows-dll
+//
+// If the library was built in c++2b mode, this test would succeed.
+//
+// Older CMake passed -std:c++latest to set C++ 20 mode on clang-cl, which
+// hid this issue. With newer CMake versions, it passes -std:c++20 which
+// makes this fail.
+//
+// Marking as UNSUPPORTED instead of XFAIL to avoid spurious failures/successes
+// depending on the version of CMake used.
+// TODO: Remove this when the library is built in c++2b mode.
+//
+// UNSUPPORTED: windows-dll
 
 // template <class charT, class traits = char_traits<charT> >
 //   class basic_ostream;