Fix non_copy_const ICE

[Jarcho]

fixes #15069

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changelog: none

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[llogiq]

I guess the only question is why did we use that binder+instantiate rigmarole in the first place?

Anyway, LGTM.

[Jarcho]

It's needed when reading types from init_typeck since they come from a different body. I accidentally applied it in that spot as well.