Auto merge of #67476 - mark-i-m:simplify-borrow_check-5, r=matthewjasper

Region naming refactoring [6/N]

Followup to #67474

EDIT: this PR is probably best read commit-by-commit...

The major changes in this PR include:
- moving many functions around to modules that better suit them. In particular, a lot of methods were moved from `borrow_check::diagnostics::region_errors` to `borrow_check::region_infer`, and `report_region_errors` was moved from `borrow_check` to `borrow_check::diagnostics::region_errors`.
- `borrow_check::diagnostics::{region_errors, region_name}` are now most comprised of methods on `MirBorrowckCtxt` instead of `RegionInferenceContext`, allowing us to get rid of the annoying `pub(in crate::borrow_check)` on most of the fields of the latter, along with a number of method arguments on many methods.
- I renamed `MirBorrowckCtxt.nonlexical_regioncx` to just `regioncx` because their is no lexical lifetimes any more, and the old name was annoyingly verbose, causing many lines to wrap unnecessarily.
- I got rid of `ErrorRegionNamingContext`. Region naming is implemented as inherent methods on `MirBorrowckCtxt`, so we just move the naming stuff into that struct.

The PR is rather large, but the commits are fairly self-contained (though they don't all compile). There was one minor output change to one test with `compare-mode=nll`, which I think is acceptable.

Between this PR and the last one, a net of 200 lines are removed, most of which was function parameters and context structs 🎉

Some samples:

```diff
-                        self.nonlexical_regioncx.free_region_constraint_info(
-                            &self.body,
-                            &self.local_names,
-                            &self.upvars,
-                            self.mir_def_id,
-                            self.infcx,
-                            borrow_region_vid,
-                            region,
-                        );
+                        self.free_region_constraint_info(borrow_region_vid, region);
```

```diff
-            .or_else(|| {
-                self.give_name_if_anonymous_region_appears_in_yield_ty(
-                    infcx,
-                    body,
-                    *mir_def_id,
-                    fr,
-                    renctx,
-                )
-            });
+            .or_else(|| self.give_name_if_anonymous_region_appears_in_arguments(fr))
```

r? @matthewjasper

cc @eddyb

src/test/ui/c-variadic/variadic-ffi-4.nll.stderr

@@ -87,12 +87,12 @@ error[E0597]: `ap1` does not live long enough
   --> $DIR/variadic-ffi-4.rs:24:11
    |
 LL | pub unsafe extern "C" fn no_escape4(_: usize, ap0: &mut VaListImpl, mut ap1: ...) {
-   |                                                    - let's call the lifetime of this reference `'1`
+   |                                                    - let's call the lifetime of this reference `'3`
 LL |     ap0 = &mut ap1;
    |     ------^^^^^^^^
    |     |     |
    |     |     borrowed value does not live long enough
-   |     assignment requires that `ap1` is borrowed for `'1`
+   |     assignment requires that `ap1` is borrowed for `'3`
 ...
 LL | }
    | - `ap1` dropped here while still borrowed