Make use of new `outer_expn_info()` function

[phansch]

Just so we don't forget about it: rust-lang/rust#61253 (comment)

the new outer_expn_info() function introduced in the second commit can be used in Clippy -- every single occurence of outer() in Clippy is immediately followed by expn_info(). (Note that some of these occurrences span two lines, and won't be found with a simple grep.) It's possible that using the new function might speed up Clippy.

Once the PR is merged, we should replace outer().expn_info() with outer_expn_info().