[ORC] Thread Error/Expected through the RPC library.

[ORC] Thread Error/Expected through the RPC library.

This replaces use of std::error_code and ErrorOr in the ORC RPC support library
with Error and Expected. This required updating the OrcRemoteTarget API, Client,
and server code, as well as updating the Orc C API.

This patch also fixes several instances where Errors were dropped.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@267457 91177308-0d34-0410-b5e6-96231b3b80d8