Do not show negative polarity trait implementations in diagnostic messages for similar implementations

[ThePuzzlemaker]

This fixes #79458.

Previously, this code:

#[derive(Clone)]
struct Foo<'a, T> {
    x: &'a mut T,
}

would have suggested that <&mut T as Clone> was an implementation that was found. This is due to the fact that the standard library now has impl<'_, T> !Clone for &'_ mut T, and explicit negative polarity implementations were not filtered out in diagnostic output when suggesting similar implementations.

This PR fixes this issue by filtering out negative polarity trait implementations in find_similar_impl_candidates within rustc_trait_selection::traits::error_reporting::InferCtxtPrivExt<'tcx>. It also adds a UI regression test for this issue and fixes UI tests that had incorrectly been modified to expect the invalid output.

r? @scottmcm

[ThePuzzlemaker]

Failures were due to GHA, it says there's a check error on .github#L1

[scottmcm]

Thanks for the PR! I re-kicked the actions and it looks like they worked. Great to get all that other test output cleaned up too.

@bors r+

[bors]

📌 Commit b287806 has been approved by scottmcm

[bors]

⌛ Testing commit b287806 with merge d37afad...

[bors]

☀️ Test successful - checks-actions
Approved by: scottmcm
Pushing d37afad to master...