Adds support for the Error class, Expected class template, and related
APIs that will be used for error propagation and handling in the new ORC
runtime.
The implementations of these types are cut-down versions of similar APIs
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.
Ported from the old ORC runtime at compiler-rt/lib/orc.
3ce25abd4a