Issue an error when using `no_mangle` on language items This pull requests adds the code to issue an error or a warning when using `no_mangle` on language items. This should detail why the `undefined symbol` error is issued for the code described in #139923. The pull request adds two ui tests, one testing the error and the other one the warning. I would love some feedback here, as I am not sure that the error and warning are issues using the right API.
Please read the rustc-dev-guide chapter on Backend Agnostic Codegen.