make codemap more robust in face of ill-formed spans.

This can be considered partial work on #8256.

The main observable change: macro expansion sometimes results in spans
where `lo > hi`; so for now, when we have such a span, do not attempt
to return a snippet result.

(Longer term, we might think about whether we could still present a
snippet for the cases where this arises, e.g. perhaps by showing the
whole macro as the snippet, assuming that is the sole cause of such
spans; or by somehow looking up the closest AST node that holds both
`lo` and `hi`, and showing that.)

As a drive-by, revised the API to return a `Result` rather than an
`Option`, with better information-packed error value that should help
us (and maybe also our users) identify the causes of such problems in
the future.  Ideally the call-sites that really want an actual snippet
would be updated to catch the newly added `Err` case and print
something meaningful about it, but that is not part of this PR.

src/libsyntax/parse/mod.rs

@@ -1233,8 +1233,8 @@ mod test {
         let span = tts.iter().rev().next().unwrap().get_span();
 
         match sess.span_diagnostic.cm.span_to_snippet(span) {
-            Some(s) => assert_eq!(&s[], "{ body }"),
-            None => panic!("could not get snippet"),
+            Ok(s) => assert_eq!(&s[], "{ body }"),
+            Err(_) => panic!("could not get snippet"),
         }
     }
 }