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_trait_selection/src/traits/coherence.rs

@@ -535,7 +535,10 @@ fn plug_infer_with_placeholders<'tcx>(
                         ct,
                         ty::Const::new_placeholder(
                             self.infcx.tcx,
-                            ty::Placeholder { universe: self.universe, bound: self.next_var() },
+                            ty::Placeholder {
+                                universe: self.universe,
+                                bound: ty::BoundConst { var: self.next_var() },
+                            },
                         ),
                     )
                 else {