Fix deadlock in Python one-line execution.

Python one-line execution was using ConnectionFileDescriptor to do
a non-blocking read against a pipe.  This won't work on Windows,
as CFD is implemented using select(), and select() only works with
sockets on Windows.

The solution is to use ConnectionGenericFile on Windows, which uses
the native API to do overlapped I/O on the pipe.  This in turn
requires re-implementing Host::Pipe on Windows using native OS
handles instead of the more portable _pipe CRT api.

Reviewed by: Greg Clayton
Differential Revision: http://reviews.llvm.org/D5679