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_error_messages/src/lib.rs

@@ -354,6 +354,13 @@ impl DiagnosticMessage {
             }
         }
     }
+
+    pub fn as_str(&self) -> Option<&str> {
+        match self {
+            DiagnosticMessage::Eager(s) | DiagnosticMessage::Str(s) => Some(s),
+            DiagnosticMessage::FluentIdentifier(_, _) => None,
+        }
+    }
 }
 
 impl From<String> for DiagnosticMessage {