Add host layer support for pipes.

Windows does support pipes, but they do so in a slightly different way. Added a Host layer which abstracts the use of pipes into a new Pipe class that everyone can use.

Windows benefits include:
- Being able to interrupt running processes when IO is directly hooked up 
- being able to interrupt long running python scripts
- being able to interrupt anything based on ConnectionFileDescriptor

llvm-svn: 212220