Bail out when encountering likely missing turbofish in parser

When encountering a likely intended turbofish without `::`, bubble
up the diagnostic instead of emitting it to allow the parser to recover
more gracefully and avoid uneccessary type errors that are likely to be
wrong.

src/libsyntax/parse/parser/expr.rs

@@ -231,7 +231,7 @@ impl<'a> Parser<'a> {
 
             self.bump();
             if op.is_comparison() {
-                self.check_no_chained_comparison(&lhs, &op);
+                self.check_no_chained_comparison(&lhs, &op)?;
             }
             // Special cases:
             if op == AssocOp::As {