Take &mut Diagnostic in emit_diagnostic.

Taking a Diagnostic by move would break the usual pattern
`diag.label(..).emit()`.

Author: cjgillot

src/driver.rs

@@ -178,8 +178,8 @@ fn report_clippy_ice(info: &panic::PanicInfo<'_>, bug_report_url: &str) {
     // a .span_bug or .bug call has already printed what
     // it wants to print.
     if !info.payload().is::<rustc_errors::ExplicitBug>() {
-        let d = rustc_errors::Diagnostic::new(rustc_errors::Level::Bug, "unexpected panic");
-        handler.emit_diagnostic(&d);
+        let mut d = rustc_errors::Diagnostic::new(rustc_errors::Level::Bug, "unexpected panic");
+        handler.emit_diagnostic(&mut d);
     }
 
     let version_info = rustc_tools_util::get_version_info!();