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.

src/parse/session.rs

@@ -162,6 +162,7 @@ fn default_handler(
             ignore_path_set,
             can_reset,
         }),
+        None,
     )
 }
 
@@ -233,7 +234,7 @@ impl ParseSess {
     }
 
     pub(crate) fn set_silent_emitter(&mut self) {
-        self.parse_sess.span_diagnostic = Handler::with_emitter(true, None, silent_emitter());
+        self.parse_sess.span_diagnostic = Handler::with_emitter(true, None, silent_emitter(), None);
     }
 
     pub(crate) fn span_to_filename(&self, span: Span) -> FileName {