Auto merge of #57609 - matthewjasper:more-restrictive-match, r=pnkfelix

Auto merge of #57609 - matthewjasper:more-restrictive-match, r=pnkfelix

Use normal mutable borrows in matches

`ref mut` borrows are currently two-phase with NLL enabled. This changes them to be proper mutable borrows. To accommodate this, first the position of fake borrows is changed:

```text
[ 1. Pre-match ]
       |
[ (old create fake borrows) ]
[ 2. Discriminant testing -- check discriminants ] <-+
       |                                             |
       | (once a specific arm is chosen)             |
       |                                             |
[ (old read fake borrows) ]                          |
[ 3. Create "guard bindings" for arm ]               |
[ (create fake borrows) ]                            |
       |                                             |
[ 4. Execute guard code ]                            |
[ (read fake borrows) ] --(guard is false)-----------+
       |
       | (guard results in true)
       |
[ 5. Create real bindings and execute arm ]
       |
[ Exit match ]
```

The following additional changes are made to accommodate `ref mut` bindings:

* We no longer create fake `Shared` borrows. These borrows are no longer needed for soundness, just to avoid some arguably strange cases.
* `Shallow` borrows no longer conflict with existing borrows, avoiding conflicting access between the guard borrow access and the `ref mut` borrow.

There is some further clean up done in this PR:

* Avoid the "later used here" note for Shallow borrows (since it's not relevant with the message provided)
* Make any use of a two-phase borrow activate it.
* Simplify the cleanup_post_borrowck passes into a single pass.

cc #56254

r? @nikomatsakis