[libcxx] [test] Fix tests of <cuchar> that unexpectedly succeed on windows

[mstorsjo]

Summary

The tests expect that the <cuchar> include should fail. When libc++
is built on top of the MSVC runtime, the header does exist provided
by MSVC. Therefore, just mark the test as unsupported on windows,
to avoid tests that unexpectedly succeed.

[• Quuxplusone]

D97870 is related.

[Mordante]

Maybe update the comment that these changes can be removed in D97870. Other than that LGTM.

[Mordante]

Sorry accidentally accepted for libc++.

[mstorsjo]

Reuploading to clear the "needs revision" status

[mstorsjo]

... now also amended with the requested comment.

[ldionne]

LGTM. Sorry, didn't mean to block this on D97870.

libcxx/test/libcxx/strings/c.strings/version_cuchar.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // XFAIL: *
 
+// Skip this test on windows. If built on top of the MSVC runtime, the
+// <cuchar> header actually does exist (although not provided by us).
+// This should be removed once D97870 has landed.
+// UNSUPPORTED: windows
+
 // <cuchar>
 
 #include <cuchar>
 
 #include "test_macros.h"
 
 #ifndef _LIBCPP_VERSION
 #error _LIBCPP_VERSION not defined
 #endif
 
 int main(int, char**)
 {
 
   return 0;
 }

libcxx/test/std/strings/c.strings/cuchar.pass.cpp

 //===----------------------------------------------------------------------===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // XFAIL: libc++
 
+// Skip this test on windows. If built on top of the MSVC runtime, the
+// <cuchar> header actually does exist (although not provided by us).
+// This should be removed once D97870 has landed.
+// UNSUPPORTED: windows
+
 // <cuchar>
 
 #include <cuchar>
 
 #include "test_macros.h"
 
 int main(int, char**)
 {
 
   return 0;
 }