Revert "[ORC][RPC] Make the future type of an Orc RPC call Error/Expected rather than"

This reverts commit r280016, and the followups of r280017, r280027,
r280051, r280058, and r280059.

MSVC's implementation of std::promise does not get along with
llvm::Error. It uses its promised value too much like a normal value
type.

llvm-svn: 280100