Simplify calls to tcx.mk_const

`mk_const(ty::ConstKind::X(...), ty)` can now be simplified to
`mk_cosnt(...,                   ty)`.

I searched with the following regex: \mk_const\([\n\s]*(ty::)?ConstKind\
I've left `ty::ConstKind::{Bound, Error}` as-is, they seem clearer this
way.

compiler/rustc_trait_selection/src/traits/project.rs

@@ -818,7 +818,7 @@ impl<'tcx> TypeFolder<'tcx> for BoundVarReplacer<'_, 'tcx> {
                 let universe = self.universe_for(debruijn);
                 let p = ty::PlaceholderConst { universe, name: bound_const };
                 self.mapped_consts.insert(p, bound_const);
-                self.infcx.tcx.mk_const(ty::ConstKind::Placeholder(p), ct.ty())
+                self.infcx.tcx.mk_const(p, ct.ty())
             }
             _ => ct.super_fold_with(self),
         }