Improve spans on evaluated `cfg_attr`s.
When converting something like `#![cfg_attr(cond, attr)]` into `#![attr]`, we currently duplicate the `#` token and the `!` token. But weirdly, there is also this comment:
// We don't really have a good span to use for the synthesized `[]`
// in `#[attr]`, so just use the span of the `#` token.
Maybe that comment used to be true? But now it is false: we can duplicate the existing delimiters (and their spans and spacing), much like we do for the `#` and `!`.
This commit does that, thus removing the incorrect comment, and improving the spans on `Group`s in a few proc-macro tests.
`@petrochenkov`
2dc88bf88a