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

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.

Differential Revision: https://reviews.llvm.org/D99096

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

@@ -8,6 +8,11 @@
 //
 // 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>

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

@@ -8,6 +8,11 @@
 //
 // 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>