[libc] Make the RPC interfaces move only

This patch uses the changed interface in D149972 to make these classes
move-only. The `Port` class could be further refined to be
construct-only in a future patch, but for now this makes it more
difficult to misuse the interface.

Depends on D149972

Reviewed By: JonChesterfield

Differential Revision: https://reviews.llvm.org/D149974