Auto merge of #117908 - lcnr:region-kind-rename, r=BoxyUwU

finish `RegionKind` renaming

second step of https://github.com/rust-lang/types-team/issues/95

continues the work from #117876. While working on this and I encountered a bunch of further cleanup which I'll either open a tracking issue for or will do in a separate PR:
- rewrite the `RegionKind` docs, they still talk about `ReEmpty` and are generally out of date
- rename `DescriptionCtx` to `DescriptionCtxt`
- what is `CheckRegions::Bound`?
- `collect_late_bound_regions` et al
- `erase_late_bound_regions` -> `instantiate_bound_regions_with_erased`?
- `EraseEarlyRegions` visitor should be removed, feels duplicate

r? `@BoxyUwU`

compiler/rustc_smir/src/rustc_smir/mod.rs

@@ -9,7 +9,7 @@
 
 use crate::rustc_internal::{IndexMap, RustcInternal};
 use crate::rustc_smir::hir::def::DefKind;
-use crate::rustc_smir::stable_mir::ty::{BoundRegion, EarlyBoundRegion, Region};
+use crate::rustc_smir::stable_mir::ty::{BoundRegion, EarlyParamRegion, Region};
 use rustc_hir as hir;
 use rustc_middle::mir;
 use rustc_middle::mir::interpret::{alloc_range, AllocId};
@@ -1729,7 +1729,7 @@ impl<'tcx> Stable<'tcx> for ty::RegionKind<'tcx> {
     fn stable(&self, tables: &mut Tables<'tcx>) -> Self::T {
         use stable_mir::ty::RegionKind;
         match self {
-            ty::ReEarlyBound(early_reg) => RegionKind::ReEarlyBound(EarlyBoundRegion {
+            ty::ReEarlyParam(early_reg) => RegionKind::ReEarlyParam(EarlyParamRegion {
                 def_id: tables.region_def(early_reg.def_id),
                 index: early_reg.index,
                 name: early_reg.name.to_string(),