[libcxx] [test] Convert an XFAIL LIBCXX-WINDOWS-FIXME into UNSUPPORTED with explanation

[Mordante]

LGTM. It seems the build errors aren't yours, but part of the GCC 11 migration.

libcxx/test/libcxx/debug/extern-templates.sh.cpp

 // -*- C++ -*-
 //===----------------------------------------------------------------------===//
 //
 // 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
 //
 //===----------------------------------------------------------------------===//
 
 // This test checks that we retain extern template instantiation declarations
 // for members of <locale> even when the debug mode is enabled, which is
 // necessary for correctness. See https://llvm.org/D94718 for details.
 
 // UNSUPPORTED: libcxx-no-debug-mode
 // UNSUPPORTED: libcpp-has-no-localization
 
-// XFAIL: LIBCXX-WINDOWS-FIXME
+// This test relies on linking a shared library and then passing that shared
+// library as input when linking an executable; this is generally not supported
+// on Windows (GNU ld supports it, but MS link.exe and LLD don't) - one has to
+// use an import library instead. (Additionally, the test uses the -fPIC
+// option which clang doesn't accept on Windows.)
+// UNSUPPORTED: windows
 
 // RUN: %{cxx} %{flags} %{compile_flags} %s %{link_flags} -fPIC -DTU1 -D_LIBCPP_DEBUG=1 -fvisibility=hidden -shared -o %t.lib
 // RUN: cd %T && %{cxx} %{flags} %{compile_flags} %s ./%basename_t.tmp.lib %{link_flags} -fPIC -DTU2 -D_LIBCPP_DEBUG=1 -fvisibility=hidden -o %t.exe
 // RUN: %{exec} %t.exe
 
 #include <cassert>
 #include <cstdio>
 #include <sstream>