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.

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