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_infer/src/infer/mod.rs

@@ -2049,10 +2049,10 @@ fn replace_param_and_infer_substs_with_placeholder<'tcx>(
                     bug!("const `{ct}`'s type should not reference params or types");
                 }
                 tcx.mk_const(
-                    ty::ConstKind::Placeholder(ty::PlaceholderConst {
+                    ty::PlaceholderConst {
                         universe: ty::UniverseIndex::ROOT,
                         name: ty::BoundVar::from_usize(idx),
-                    }),
+                    },
                     ty,
                 )
                 .into()