Auto merge of #107844 - Zeegomo:no-drop-and-rep, r=cjgillot

Desugaring of drop and replace at MIR build

This commit desugars the drop and replace deriving from an
assignment at MIR build, avoiding the construction of the
`DropAndReplace` terminator (which will be removed in a following PR).

In order to retain the same error messages for replaces a new
`DesugaringKind::Replace` variant is introduced.

The changes in the borrowck are also useful for future work in moving drop elaboration
before borrowck, as no `DropAndReplace` would be present there anymore.

Notes on test diffs:
*  `tests/ui/borrowck/issue-58776-borrowck-scans-children`: the assignment deriving from the desugaring kills the borrow.
*  `tests/ui/async-await/async-fn-size-uninit-locals.rs`, `tests/mir-opt/issue_41110.test.ElaborateDrops.after.mir`,  `tests/mir-opt/issue_41888.main.ElaborateDrops.after.mir`:  drop elaboration generates (or reads from) a useless drop flag due to an issue with the dataflow analysis. Will be fixed independently by rust-lang/rust#106430.

See rust-lang/rust#104488 for more context

Author: bors