Fix presentation of purely "additive" replacement suggestion parts
#127541 changes replacement suggestions to use the "diff" view always, which I think is really verbose in cases where a replacement snippet is a "superset" of the snippet that is being replaced.
Consider:
```
LL - Self::Baz: Clone,
LL + Self::Baz: Clone, T: std::clone::Clone
```
In this code, we suggest replacing `", "` with `", T: std::clone::Clone"`. This is a consequence of how the snippet is constructed. I believe that since the string that is being replaced is a subset of the replacement string, it's not providing much value to present this as a diff. Users should be able to clearly understand what's being suggested here using the `~` underline view we've been suggesting for some time now.
Given that this affects ~100 tests out of the ~1000 UI tests affected, I expect this to be a pretty meaningful improvement of the fallout of #127541.
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In the last commit, this PR also "trims" replacement parts so that they are turned into their purely additive subset, if possible. See the diff for what this means.
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r? estebank
49fb61c496