[libc++] Flag file_clock test as expected to fail with ASAN

This silences a known issue, as can be seen by looking at similar
tests for other clocks, like time.clock.steady/consistency.pass.cpp.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@356528 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed