Use ipv4 localhost address in lldb-server tests

Since the ipv6 patch, we've experienced occasional flakyness in
lldb-server tests. This was due to the fact that lldb-server was trying
to listen on both v4 and v6 localhost sockets (and consider it a success
if at least one of them succeeded), while the test framework was only
trying to connect to the v4 one.

This change makes sure lldb-server only listens on the v4 socket.

llvm-svn: 316391