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.

compiler/rustc_expand/src/tests.rs

@@ -161,7 +161,7 @@ fn test_harness(file_text: &str, span_labels: Vec<SpanLabel>, expected_output: &
             false,
             TerminalUrl::No,
         );
-        let handler = Handler::with_emitter(true, None, Box::new(emitter));
+        let handler = Handler::with_emitter(true, None, Box::new(emitter), None);
         #[allow(rustc::untranslatable_diagnostic)]
         handler.span_err(msp, "foo");