Test for both IPv4 and IPv6 support to determine if either `127.0.0.1` or `[::1]` are appropriate IP addresses to attempt to connect to. In an IPv6-only environment, `127.0.0.1` is not available.
Using `localhost` is problematic because we might not be able to get the same port on each IP flavor, and later try to connect to the wrong thing.
Reviewed By: labath
Differential Revision: https://reviews.llvm.org/D133393
Our (TCP) socket support is in a much better state than pipes. Use that
for testing the Communication class.
Move the CreateTCPConnectedSockets function
(SocketTestUtilities.{h,cpp}) to a place where it can be used from
Communication tests.
