Ensure that SpanData::cmp returns Equal whenever PartialEq does

Author: oli-obk

compiler/rustc_span/src/lib.rs

@@ -480,6 +480,9 @@ pub struct SpanData {
 // Order spans by position in the file.
 impl Ord for SpanData {
     fn cmp(&self, other: &Self) -> Ordering {
+        if self == other {
+            return Ordering::Equal;
+        }
         let SpanData {
             lo: s_lo,
             hi: s_hi,