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

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@257452 91177308-0d34-0410-b5e6-96231b3b80d8