Rollup merge of #122080 - Zalathar:drop-tree, r=oli-obk

Clarity improvements to `DropTree`

These changes are based on some points of confusion I had when initially trying to understand this code.

The only “functional” change is an additional assertion in `<ExitScopes as DropTreeBuilder>::link_entry_point`, checking that the dummy terminator is `TerminatorKind::UnwindResume` as expected.

Author: workingjubilee