Rename the ConstS::val field as kind.

And likewise for the `Const::val` method.

Because its type is called `ConstKind`. Also `val` is a confusing name
because `ConstKind` is an enum with seven variants, one of which is
called `Value`. Also, this gives consistency with `TyS` and `PredicateS`
which have `kind` fields.

The commit also renames a few `Const` variables from `val` to `c`, to
avoid confusion with the `ConstKind::Value` variant.

compiler/rustc_lint/src/builtin.rs

@@ -2873,7 +2873,7 @@ impl ClashingExternDeclarations {
                         }
                         (Array(a_ty, a_const), Array(b_ty, b_const)) => {
                             // For arrays, we also check the constness of the type.
-                            a_const.val() == b_const.val()
+                            a_const.kind() == b_const.kind()
                                 && structurally_same_type_impl(seen_types, cx, *a_ty, *b_ty, ckind)
                         }
                         (Slice(a_ty), Slice(b_ty)) => {