[libcxx] don't #include <cwchar> if wide chars aren't enabled

Pull request llvm#96032 unconditionall adds the `cwchar` include in the `format`
umbrella header. However support for wchar_t can be disabled in the build
system (LIBCXX_ENABLE_WIDE_CHARACTERS).

This patch guards against inclusion of `cwchar` in `format` by checking the
`_LIBCPP_HAS_NO_WIDE_CHARACTERS` define.

libcxx/include/format

@@ -237,21 +237,21 @@ namespace std {
 #  include <cstdint>
 #  include <cstdlib>
 #  include <cstring>
-#  include <cwchar>
 #  include <initializer_list>
 #  include <limits>
+#  include <locale>
 #  include <new>
 #  include <optional>
+#  include <queue>
+#  include <stack>
 #  include <stdexcept>
 #  include <string>
 #  include <string_view>
 #  include <tuple>
-#endif
 
-#if !defined(_LIBCPP_REMOVE_TRANSITIVE_INCLUDES) && _LIBCPP_STD_VER <= 20
-#  include <locale>
-#  include <queue>
-#  include <stack>
+#if !defined(_LIBCPP_HAS_NO_WIDE_CHARACTERS)
+#  include <cwchar>
+#endif
 #endif
 
 #endif // _LIBCPP_FORMAT