libsyntax: Forbid type parameters in tuple indices

This breaks code like

```
let t = (42i, 42i);
... t.0::<int> ...;
```

Change this code to not contain an unused type parameter. For example:

```
let t = (42i, 42i);
... t.0 ...;
```

Closes https://github.com/rust-lang/rust/issues/19096

[breaking-change]

r? @aturon