Record span of const kw in GenericParamKind

Context: this is needed to fix rust-lang/rustfmt#4263,
which currently records the span of a const generic param incorrectly
because the location of the `const` kw is not known.

I am not sure how to add tests for this; any guidance in how to do so
would be appreciated 🙂

clippy_lints/src/utils/ast_utils.rs

@@ -476,7 +476,7 @@ pub fn eq_generic_param(l: &GenericParam, r: &GenericParam) -> bool {
         && match (&l.kind, &r.kind) {
             (Lifetime, Lifetime) => true,
             (Type { default: l }, Type { default: r }) => both(l, r, |l, r| eq_ty(l, r)),
-            (Const { ty: l }, Const { ty: r }) => eq_ty(l, r),
+            (Const { ty: l, kw_span: _ }, Const { ty: r, kw_span: _ }) => eq_ty(l, r),
             _ => false,
         }
         && over(&l.attrs, &r.attrs, |l, r| eq_attr(l, r))