Auto merge of #113801 - compiler-errors:iter-instantiated, r=oli-obk

Rename `arg_iter` to `iter_instantiated`

`arg_iter` doesn't make sense, and doesn't really indicate what it's doing (returning an iterator that ~~substitutes~~ instantiates its elements).

`iter_instantiated_copied` is kinda awkward but i don't really wanna bikeshed it.

r? `@oli-obk`