Rollup merge of #151863 - amandasystems:streamline-borrow-error-handling, r=lcnr

Borrowck: simplify diagnostics for placeholders

This folds the call to `region_from_element` into `RegionInferenceContext`, and simplifies the error variant for this case to only talk about regions as opposed to elements. This is the only case where a `RegionElement` leaks out of region inference, so now they can be considered internal to region inference (though that currently isn't expressed). It also clarifies the type information on the methods called to emphasise the fact that they only ever use placeholder regions in the diagnostics completely ignore any other element.

It also adds a bunch of FIXMEs to some fishy statements that conjure universes from what seems like arbitrary integers.

This was lifted from rust-lang/rust#142623.

r? @lcnr

Author: jhpratt