These should have been removed in rust-lang#127233 when the positions were
changed from `usize` to `u32`.

To distinguish it from the `HasTokens` method.

The only place it is meaningfully used is in a panic message in
`TokenStream::from_ast`. But `node.span()` doesn't need to be printed
because `node` is also printed and it must contain the span.

Currently the second element is a `Vec<(FlatToken, Spacing)>`. But the
vector always has zero or one elements, and the `FlatToken` is always
`FlatToken::AttrTarget` (which contains an `AttributesData`), and the
spacing is always `Alone`. So we can simplify it to
`Option<AttributesData>`.

An assertion in `to_attr_token_stream` can can also be removed, because
`new_tokens.len()` was always 0 or 1, which means than `range.len()`
is always greater than or equal to it, because `range.is_empty()` is
always false (as per the earlier assertion).

- `AttributesData` -> `AttrsTarget`
- `AttrTokenTree::Attributes` -> `AttrTokenTree::AttrsTarget`
- `FlatToken::AttrTarget` -> `FlatToken::AttrsTarget`

`Option<LazyAttrTokenStream>` is the type that's actually used in all
the aST nodes.

All the branches produce either zero or one elements.