Fix last_write_time.pass.cpp to work with clang-3.9 and earlier

At least with clang-3.9 and earlier, -Wunknown-pragmas is also needed.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@315882 91177308-0d34-0410-b5e6-96231b3b80d8