[Support] Make all Errors convertible to std::error_code.

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.

llvm-svn: 264221