Auto merge of #90281 - xldenis:public-borrow-set, r=nikomatsakis

Auto merge of #90281 - xldenis:public-borrow-set, r=nikomatsakis

Add BorrowSet to public api

This PR adds `BorrowSet` to the public api so that verification tools can obtain the activation and reservation points of two phase borrows without having to redo calculations themselves (and thus potentially differently from rustc).

Turns out we already can obtain `MoveData` thanks to the public `HasMoveData` trait, so constructing a `BorrowSet` should not provide much of an issue. However, I can't speak to the soundness of this approach, is it safe to take an under-approximation of `MoveData`?

r? `@nikomatsakis`