On nightly, dump ICE backtraces to disk

Implement rust-lang/compiler-team#578.

When an ICE is encountered on nightly releases, the new rustc panic
handler will also write the contents of the backtrace to disk. If any
`delay_span_bug`s are encountered, their backtrace is also added to the
file. The platform and rustc version will also be collected.

Author: estebank

clippy_lints/src/doc.rs

@@ -729,7 +729,7 @@ fn check_code(cx: &LateContext<'_>, text: &str, edition: Edition, span: Span) {
                     false,
                     TerminalUrl::No,
                 );
-                let handler = Handler::with_emitter(false, None, Box::new(emitter));
+                let handler = Handler::with_emitter(false, None, Box::new(emitter), None);
                 let sess = ParseSess::with_span_handler(handler, sm);
 
                 let mut parser = match maybe_new_parser_from_source_str(&sess, filename, code) {