Just pass Layout directly to box_new_uninit

We have a constant for it already (used in `RawVec` for basically the same polymorphization) so let's use it.

Conveniently, it can even be safe that way!

tests/codegen-units/item-collection/opaque-return-impls.rs

@@ -45,8 +45,6 @@ pub fn foo2() -> Box<dyn TestTrait2> {
 //~ MONO_ITEM fn foo2
 //~ MONO_ITEM fn std::alloc::Global::alloc_impl_runtime
 //~ MONO_ITEM fn std::boxed::Box::<TestStruct2>::new
-//~ MONO_ITEM fn std::alloc::Layout::from_size_align_unchecked::precondition_check
-//~ MONO_ITEM fn std::ptr::Alignment::new_unchecked::precondition_check
 //~ MONO_ITEM fn std::ptr::NonNull::<T>::new_unchecked::precondition_check
 
 struct Counter {