Rollup merge of #151696 - amandasystems:scc-annotations-update, r=lcnr

Borrowck: Simplify SCC annotation computation, placeholder rewriting

This change backports some changes from the now abandoned rust-lang/rust#142623.

Notably, it simplifies the `PlaceholderReachability` `enum` by replacing the case when no placeholders were reached with a standard `Option::None`.

It also rewrites the API for `scc::Annotations` to be update-mut rather than a more Functional programming style. This showed some slight performance impact in early tests of the PR and definitely makes the implementation simpler.

This probably wants a perf run just for good measure.

r? @lcnr

Author: JonathanBrouwer