save-analysis: be more paranoid about generated paths

fixes https://github.com/rust-lang-nursery/rls/issues/160

src/libsyntax/parse/parser.rs

@@ -1693,6 +1693,7 @@ impl<'a> Parser<'a> {
         }
 
         // Assemble the span.
+        // FIXME(#39450) This is bogus if part of the path is macro generated.
         let span = mk_sp(lo, self.prev_span.hi);
 
         // Assemble the result.