Print port number that lldb-server binds to in platform mode

If binding to port 0 is selected, the actual port is printed.

This improves the reliability of platform startup by ensuring that
a free port can be found.

TEST PLAN
./lldb-server platform --listen *:0

Listening for a connection from <port-number>...

Will appear on stdout (with other stuff potentially)

llvm-svn: 238173