This will be used as a replacement for selecting over a pipe fd, which
does not work on windows. The posix implementation still uses a pipe
under the hood, while the windows version uses windows event handles.
The idea is that, instead of writing to a pipe, one just inserts a
callback, which does whatever you wanted to do after the bytes come out
the read end of the pipe.
Differential Revision: https://reviews.llvm.org/D131160
6a8bbd26ab