[libcxx] [test] Remove XFAIL LIBCXX-WINDOWS-FIXME from time.clock.file/now.pass.cpp

[mstorsjo]

Summary

This doesn't fail when _LIBCPP_HAS_NO_INT128 is defined consistently
in both CMAKE_CXX_FLAGS and LIBCXX_TEST_COMPILER_FLAGS; the XFAIL was
added based on early CI testruns where that flag was missing in
LIBCXX_TEST_COMPILER_FLAGS.

[curdeius]

LGTM.
Ceterum autem censeo, Windows'em esse testendam, we should have Windows CI. :)

libcxx/test/std/utilities/time/time.clock/time.clock.file/now.pass.cpp

 // Filesystem is supported on Apple platforms starting with macosx10.15.
 // UNSUPPORTED: with_system_cxx_lib=macosx10.14
 // UNSUPPORTED: with_system_cxx_lib=macosx10.13
 // UNSUPPORTED: with_system_cxx_lib=macosx10.12
 // UNSUPPORTED: with_system_cxx_lib=macosx10.11
 // UNSUPPORTED: with_system_cxx_lib=macosx10.10
 // UNSUPPORTED: with_system_cxx_lib=macosx10.9
 
-// XFAIL: LIBCXX-WINDOWS-FIXME
-
 // <chrono>
 
 // file_clock
 
 // static time_point now() noexcept;
 
 #include <chrono>
 #include <cassert>