Remove good_path_delayed_bug.

It's only has a single remaining purpose: to ensure that a diagnostic is
printed when `trimmed_def_paths` is used. It's an annoying mechanism:
weak, with odd semantics, badly named, and gets in the way of other
changes.

This commit replaces it with a simpler `must_produce_diag` mechanism,
getting rid of a diagnostic `Level` along the way.

compiler/rustc_error_messages/src/lib.rs

@@ -376,7 +376,7 @@ impl From<Cow<'static, str>> for DiagnosticMessage {
     }
 }
 
-/// A workaround for good_path_delayed_bug ICEs when formatting types in disabled lints.
+/// A workaround for must_produce_diag ICEs when formatting types in disabled lints.
 ///
 /// Delays formatting until `.into(): DiagnosticMessage` is used.
 pub struct DelayDm<F>(pub F);