Rollup merge of #100081 - RalfJung:unused-unsafe-in-unsafe-fn, r=jackh726

never consider unsafe blocks unused if they would be required with deny(unsafe_op_in_unsafe_fn)

Judging from https://github.com/rust-lang/rust/issues/71668#issuecomment-1200317370 the consensus nowadays seems to be that we should never consider an unsafe block unused if it was required with `deny(unsafe_op_in_unsafe_fn)`, no matter whether that lint is actually enabled or not. So let's adjust rustc accordingly.

The first commit does the change, the 2nd does some cleanup.

compiler/rustc_errors/src/diagnostic_builder.rs

@@ -566,7 +566,7 @@ impl Drop for DiagnosticBuilderInner<'_> {
                         ),
                     ));
                     handler.emit_diagnostic(&mut self.diagnostic);
-                    panic!();
+                    panic!("error was constructed but not emitted");
                 }
             }
             // `.emit()` was previously called, or maybe we're during `.cancel()`.