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Auto merge of #149775 - WaffleLapkin:core-mem-maybe-dangling, r=Mark-Simulacrum
Add `MaybeDangling` to `core`
This is the library part of adding `MaybeDangling`. Note that it doesn't actually do anything described in its docs (yet), I'll make a separate PR for that.
Tracking issue: https://github.com/rust-lang/rust/issues/118166.
r? libs
cc `@RalfJung`
