[Orc] Add support for remote JITing to the ORC API.

[Orc] Add support for remote JITing to the ORC API.

This patch adds utilities to ORC for managing a remote JIT target. It consists
of:

1. A very primitive RPC system for making calls over a byte-stream.  See
RPCChannel.h, RPCUtils.h.

2. An RPC API defined in the above system for managing memory, looking up
symbols, creating stubs, etc. on a remote target. See OrcRemoteTargetRPCAPI.h.

3. An interface for creating high-level JIT components (memory managers,
callback managers, stub managers, etc.) that operate over the RPC API. See
OrcRemoteTargetClient.h.

4. A helper class for building servers that can handle the RPC calls. See
OrcRemoteTargetServer.h.

The system is designed to work neatly with the existing ORC components and
functionality. In particular, the ORC callback API (and consequently the
CompileOnDemandLayer) is supported, enabling lazy compilation of remote code.

Assuming this doesn't trigger any builder failures, a follow-up patch will be
committed which tests these utilities by using them to replace LLI's existing
remote-JITing demo code.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257305 91177308-0d34-0410-b5e6-96231b3b80d8