Sort NLL error diagnostics by span in an attempt to make them match source order

(and thus, hopefully, more closely match user expectation).

src/librustc_mir/borrow_check/mod.rs

@@ -331,6 +331,10 @@ fn do_mir_borrowck<'a, 'gcx, 'tcx>(
     }
 
     if mbcx.errors_buffer.len() > 0 {
+        mbcx.errors_buffer.sort_by(|diag1, diag2| {
+            diag1.span.primary_span().cmp(&diag2.span.primary_span())
+        });
+
         if tcx.migrate_borrowck() {
             match tcx.borrowck(def_id).signalled_any_error {
                 SignalledError::NoErrorsSeen => {