Register `~const` preds for `Deref` adjustments in HIR typeck
This doesn't *do* anything yet, since `Deref` and `DerefMut` aren't constified, and we explicitly don't error on calling non-const trait methods in HIR yet -- presumably that will wait until std is re-constified. But I'm confident this logic is correct, and this (afaict?) is the only major hole left in enforcing `~const` in HIR typeck.
r? fee1-dead
432972cae6