Make const bound handling more like types/regions.

Currently there is `Ty` and `BoundTy`, and `Region` and `BoundRegion`,
and `Const` and... `BoundVar`. An annoying inconsistency.

This commit repurposes the existing `BoundConst`, which was barely used,
so it's the partner to `Const`. Unlike `BoundTy`/`BoundRegion` it lacks
a `kind` field but it's still nice to have because it makes the const
code more similar to the ty/region code everywhere.

The commit also removes `impl From<BoundVar> for BoundTy`, which has a
single use and doesn't seem worth it.

These changes fix the "FIXME: We really should have a separate
`BoundConst` for consts".

compiler/rustc_hir_analysis/src/check/compare_impl_item.rs

@@ -2498,7 +2498,7 @@ fn param_env_with_gat_bounds<'tcx>(
                     ty::Const::new_bound(
                         tcx,
                         ty::INNERMOST,
-                        ty::BoundVar::from_usize(bound_vars.len() - 1),
+                        ty::BoundConst { var: ty::BoundVar::from_usize(bound_vars.len() - 1) },
                     )
                     .into()
                 }