These will be used for error propagation and handling in the ORC runtime.

The implementations of these types are cut-down versions of the error
support in llvm/Support/Error.h. Most advice on llvm::Error and llvm::Expected
(e.g. from the LLVM Programmer's manual) applies equally to __orc_rt::Error
and __orc_rt::Expected. The primary difference is the mechanism for testing
and handling error types: The ORC runtime uses a new 'error_cast' operation
to replace the handleErrors family of functions. See error_cast comments in
error.h.