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



git-svn-id: https://llvm.org/svn/llvm-project/lldb/trunk@212220 91177308-0d34-0410-b5e6-96231b3b80d8