Emit a diagnostic warning about parsing ambiguity in a conjunction

While parsing a compound condition with a reflect expression on the left-hand side of an `&&` operator, e.g.,
```cpp
(refl != ^T && true)
```
the `&&` token binds left to form the type `T &&`, whereas the author probably meant `refl != (^T) && true`. It would be great to detect this case and emit a warning diagnostic.