Auto merge of #152689 - scottmcm:also-simplify-of-sized-val-alt, r=cjgillot

Simplify `size/align_of_val<T: Sized>` to `size/align_of<T>` instead

This is relevant to things like `Box<[u8; 1024]>` where the drop looks at the `size_of_val` (since obviously it might be DST in general) but where we don't actually need to do that since it's always that same value for the `Sized` type.

(Equivalent to rust-lang/rust#152681, but flipped in the rebase so it can land before rust-lang/rust#152641 instead of depending on it.)

Author: bors