This is a temporary crutch to enable code that currently uses std::error_code
to be incrementally moved over to Error. Requiring all Error instances be
convertible enables clients to call errorToErrorCode on any error (not just
ECErrors created by conversion *from* an error_code).
This patch also moves code for Error from ErrorHandling.cpp into a new
Error.cpp file.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@264221 91177308-0d34-0410-b5e6-96231b3b80d8