Show clearer error message when #![deny(warnings)] escalates a warning

Addresses #30730

src/librustc/lint/context.rs

@@ -451,9 +451,14 @@ fn current_level(&self, lint: &'static Lint) -> Level {
 
     fn level_src(&self, lint: &'static Lint) -> Option<LevelSource> {
         self.lints().levels.get(&LintId::of(lint)).map(|ls| match ls {
-            &(Warn, src) => {
+            &(Warn, _) => {
                 let lint_id = LintId::of(builtin::WARNINGS);
-                (self.lints().get_level_source(lint_id).0, src)
+                let warn_src = self.lints().get_level_source(lint_id);
+                if warn_src.0 != Warn {
+                    warn_src
+                } else {
+                    *ls
+                }
             }
             _ => *ls
         })