[llvm] r210920 - Finishing touch for the std::error_code transition.

[Rafael Espíndola]

> Hrmm... okay, but this does not solve that problem, it just localizes it to this header. That being the case, why are we using the std:: names at all? Can't we use some kind of SFINAE-enhanced static table for make_error_code?

It makes sure you noticed the error, even if the code you are editing
is only used in one platform. I decided to try this once I noticed
some windows only code was using a value that was never mapped to.

Using the system values makes sure we can use generic_category.

Cheers,
Rafael