[libFuzzer] Properly use Handle instead of FD on Windows.

For Windows, sanitizers work with Handles, not with posix file descriptors,
because they use the windows-specific API. So we need to convert the fds to
handles before passing them to the sanitizer library.
After this change, close_fd_mask is fixed for Windows (this fix some tests too).

Differential Revision: https://reviews.llvm.org/D29548

llvm-svn: 294388