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- rewrite the `RegionKind` docs, they still talk about `ReEmpty` and are generally out of date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/stable_mir/src/ty.rs

@@ -61,7 +61,7 @@ pub struct Region {
 
 #[derive(Clone, Debug, Eq, PartialEq)]
 pub enum RegionKind {
-    ReEarlyBound(EarlyBoundRegion),
+    ReEarlyParam(EarlyParamRegion),
     ReBound(DebruijnIndex, BoundRegion),
     ReStatic,
     RePlaceholder(Placeholder<BoundRegion>),
@@ -71,7 +71,7 @@ pub enum RegionKind {
 pub(crate) type DebruijnIndex = u32;
 
 #[derive(Clone, Debug, Eq, PartialEq)]
-pub struct EarlyBoundRegion {
+pub struct EarlyParamRegion {
     pub def_id: RegionDef,
     pub index: u32,
     pub name: Symbol,