XFAIL Windows test failures under test/libcxx

This patch XFAIL's a number of tests under test/libcxx when on Windows.
These failures need more investigation or patches to either Clang or libc++
but for now we don't want them to prevent the bot from going green.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@300941 91177308-0d34-0410-b5e6-96231b3b80d8

test/libcxx/modules/clocale_exports.sh.cpp

@@ -7,6 +7,10 @@
 //
 //===----------------------------------------------------------------------===//
 
+// This test fails on Windows because the underlying libc headers on Windows
+// are not modular
+// XFAIL: LIBCXX-WINDOWS-FIXME
+
 // REQUIRES: modules-support
 // UNSUPPORTED: c++98, c++03