[Orc] Add overloads of RPC::handle and RPC::expect that take member functions as

handlers.

It is expected that RPC handlers will usually be member functions. Accepting them
directly in handle and expect allows for the remove of a lot of lambdas an
explicit error variables.

This patch also uses this new feature to substantially tidy up the
OrcRemoteTargetServer class.

llvm-svn: 257452