rustc_error: make ErrorReported impossible to construct

There are a few places were we have to construct it, though, and a few
places that are more invasive to change. To do this, we create a
constructor with a long obvious name.

compiler/rustc_lint/src/levels.rs

@@ -652,7 +652,7 @@ impl<'s> LintLevelsBuilder<'s> {
         &self,
         lint: &'static Lint,
         span: Option<MultiSpan>,
-        decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a>),
+        decorate: impl for<'a> FnOnce(LintDiagnosticBuilder<'a, ()>),
     ) {
         let (level, src) = self.lint_level(lint);
         struct_lint_level(self.sess, lint, level, src, span, decorate)