MGCA: Check that const arguments in struct exprs are correctly typed

[BoxyUwU]

Currently the wellformedness requirements of ty::Const does not require that ty::Const as the fields of ADTs have the type of the ADT's field. See this test

rust/tests/ui/const-generics/mgca/adt_expr_fields_type_check.rs

Lines 4 to 15 in 5497a36

	#![feature(min_generic_const_args, adt_const_params)]
	#![expect(incomplete_features)]
	#[derive(Eq, PartialEq, std::marker::ConstParamTy)]
	struct Foo<T> { field: T }
	fn accepts<const N: Foo<u8>>() {}
	fn bar<const N: bool>() {
	// `N` is not of type `u8` but we don't actually check this anywhere yet
	accepts::<{ Foo::<u8> { field: N }}>();
	}

To fix this we should update wf::obligations for ty::Const to emit ConstArgHasType goals for ty::Consts in ValTreeKind::Branch according to the type of their parent ValTree node.

cc @camelid I've assigned you as we've talked about you implementing this on zulip