regression: Rc implements RefUnwindSafe

[Mark-Simulacrum]

#87467 added an impl of RefUnwindSafe to Rc, which was detected as breaking these crates:

• https://crater-reports.s3.amazonaws.com/beta-1.58-1/beta-2021-11-30/reg/ref-portals-1.0.0-beta.2/log.txt

This seems likely to be treated as acceptable breakage (the crate in this case is specifically asserting not implementing the trait), but want to confirm that.

[m-ou-se]

Discussed briefly in the libs meeting. This is expected and acceptable.