MainLoop: avoid infinite loop when pty slave gets closed

Summary:
For ptys (at least on Linux), the end-of-file (closing of the slave FD)
is signalled by the POLLHUP flag. We were ignoring this flag, which
meant that when this happened, we would spin in a loop, continuously
calling poll(2) and not making any progress.

This makes sure we treat POLLHUP as a read event (reading will return
0), and we call the registered callback when it happens. This is the
behavior our clients expect (and is consistent with how select(2)
works).

Reviewers: eugene, beanz

Subscribers: lldb-commits

Differential Revision: https://reviews.llvm.org/D41008

llvm-svn: 320345