[MainLoop] Fix possible use of an invalid iterator

Store file descriptors from loop.m_read_fds (if FORCE_PSELECT is
defined) and signals from loop.m_signals that need to be processed in
MainLoop::RunImpl::ProcessEvents() into a separate vector and then
iterate over this container to invoke the callbacks.

This prevents a problem where when the code iterated directly over
m_read_fds/m_signals, a callback invoked from within the loop could
modify these variables and invalidate the loop iterator. This would then
result in an assertion failure in llvm::DenseMapIterator::operator++().

Differential Revision: https://reviews.llvm.org/D35298

llvm-svn: 307782