Overhaul RegionKind and Region.

Specifically, change `Region` from this:
```
pub type Region<'tcx> = &'tcx RegionKind;
```
to this:
```
pub struct Region<'tcx>(&'tcx Interned<RegionKind>);
```

This now matches `Ty` and `Predicate` more closely.

Things to note
- Regions have always been interned, but we haven't been using pointer-based
  `Eq` and `Hash`. This is now happening.
- I chose to impl `Deref` for `Region` because it makes pattern matching a lot
  nicer, and `Region` can be viewed as just a smart wrapper for `RegionKind`.
- Various methods are moved from `RegionKind` to `Region`.
- There is a lot of tedious sigil changes.
- A couple of types like `HighlightBuilder`, `RegionHighlightMode` now have a
  `'tcx` lifetime because they hold a `Ty<'tcx>`, so they can call `mk_region`.
- A couple of test outputs change slightly, I'm not sure why, but the new
  outputs are a little better.

Author: nnethercote

src/abi/mod.rs

@@ -544,7 +544,7 @@ pub(crate) fn codegen_drop<'tcx>(
                 let arg_value = drop_place.place_ref(
                     fx,
                     fx.layout_of(fx.tcx.mk_ref(
-                        &ty::RegionKind::ReErased,
+                        fx.tcx.lifetimes.re_erased,
                         TypeAndMut { ty, mutbl: crate::rustc_hir::Mutability::Mut },
                     )),
                 );