fix: don't leave trailing whitespace before a skipped impl item

[shulaoda]

Summary

Closes#5521

Review notes

Whitespace characters on the blank line before the skipped item:

written	before	after
1	0	0
2	2	0
3	2	0
4	4	0
11	10	0
12	12	0

The skipped item has to be a non-first item of an impl block. The same whitespace in a mod, in a function body, at the top level, or before the first impl item is trimmed elsewhere and was never affected, so the test uses that one shape for all four cases.

#4706 was raised on the issue as a possible duplicate. There the whitespace is inside the body of the skipped item, which the attribute asks rustfmt to leave alone. Here it sits between the previous item and the attribute, outside any skipped span.

[rustbot]

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.