Auto merge of #117880 - lqd:liveness-values, r=cjgillot

Refactor borrowck liveness values

This PR starts cleaning up `rustc_borrowck`, in particular around liveness values:
- refactors simple names that make no sense anymore: either referring to older structures using region elements, or to bitset containers and values.
- improves comments and fixes others
- removes unused return values and unneeded generic arguments

r? `@matthewjasper`