[Orc] Handle hangup messages in SimpleRemoteEPC

On the controller-side, handle `Hangup` messages from the executor. The executor passed `Error::success()` or a failure message as payload.

Hangups cause an immediate disconnect of the transport layer. The disconnect function may be called later again and so implementations should be prepared. `FDSimpleRemoteEPCTransport::disconnect()` already has a flag to check that:
https://github.com/llvm/llvm-project/blob/cd1bd95d8707371da0e4f75cd01669c427466931/llvm/lib/ExecutionEngine/Orc/Shared/SimpleRemoteEPCUtils.cpp#L112

Reviewed By: lhames

Differential Revision: https://reviews.llvm.org/D111527