[Host] File::GetWaitableHandle() should call fileno()

If the file has m_stream, it may not have a m_descriptor.
GetWaitableHandle() should call GetDescriptor(), which will call
fileno(), so it will get waitable descriptor whenever one is available.

Patch by: Lawrence D'Anna

Differential revision: https://reviews.llvm.org/D67789

llvm-svn: 372644