XFAIL a test on Windows

http://45.33.8.238/win/3052/step_6.txt

C:\src\llvm-project\clang\test\Preprocessor\file_test.c:9:11: error: CHECK: expected string not found in input
// CHECK: filename: "/UNLIKELY_PATH/empty{{/|\\\\}}file_test.c"
          ^
<stdin>:1:1: note: scanning from here
^
<stdin>:1:28: note: possible intended match here
                           ^