Auto merge of #147353 - the8472:nondrop-array-iter, r=scottmcm

only call polymorphic array iter drop machinery when the type requires it

I saw a bunch of dead, empty  `<[core::mem::maybe_uninit::MaybeUninit<T>; N] as core::array::iter::iter_inner::PartialDrop>::partial_drop` functions when compiling with more than 1 CGU.

Let's see if we can help optimizations to eliminate stuff earlier.

r? ghost

Author: bors