[lldb/ipv6] Support running lldb tests in an ipv6-only environment.

When running in an ipv6-only environment where `AF_INET` sockets are not available, many lldb tests (mostly gdb remote tests) fail because things like `127.0.0.1` don't work there.

Use `localhost` instead of `127.0.0.1` whenever possible, or include a fallback of creating `AF_INET6` sockets when `AF_INET` fails.

Reviewed By: labath

Differential Revision: https://reviews.llvm.org/D87333