Make fields of Span private

src/test/run-pass/import-crate-with-invalid-spans/main.rs

@@ -16,7 +16,7 @@ extern crate crate_with_invalid_spans;
 
 fn main() {
     // The AST of `exported_generic` stored in crate_with_invalid_spans's
-    // metadata should contain an invalid span where span.lo > span.hi.
+    // metadata should contain an invalid span where span.lo() > span.hi().
     // Let's make sure the compiler doesn't crash when encountering this.
     let _ = crate_with_invalid_spans::exported_generic(32u32, 7u32);
 }