[lldb] [Host] Move port predicate-related logic to gdb-remote

Remove the port predicate from Socket and ConnectionFileDescriptor,
and move it to gdb-remote.  It is specifically relevant to the threading
used inside gdb-remote and with the new port callback API, we can
reliably move it there.  While at it, switch from the custom Predicate
to std::promise/std::future.

Differential Revision: https://reviews.llvm.org/D112357