Change `ErrorCode` location after the `error:`

[MahadMuhammad]

The error code is after the diagnostic message, whereas rustc puts it after the word "error". I could change that in gcc's diagnostic.cc, perhaps.

Originally posted by @davidmalcolm in #1408 (comment)

Currently, the error codes are emitted after the error message in gccrs, we tried to move that after the error:. But we have a fear that it might break other languages frontend.

Funfact: Older versions of rustc also emits error message after the error message. The error code after the error: is added in rustc 1.12.0