[ORC] Add a pair of ORC layers that forward object-layer operations via RPC.

This patch introduces RemoteObjectClientLayer and RemoteObjectServerLayer,
which can be used to forward ORC object-layer operations from a JIT stack in
the client to a JIT stack (consisting only of object-layers) in the server.

This is a new way to support remote-JITing in LLVM. The previous approach
(supported by OrcRemoteTargetClient and OrcRemoteTargetServer) used a
remote-mapping memory manager that sat "beneath" the JIT stack and sent
fully-relocated binary blobs to the server. The main advantage of the new
approach is that relocatable objects can be cached on the server and re-used
(if the code that they represent hasn't changed), whereas fully-relocated blobs
can not (since the addresses they have been permanently bound to will change
from run to run).

llvm-svn: 312511