Rename various "concrete opaque type" things to say "hidden type"
r? lcnr
I've found "concrete opaque type" terminology to be somewhat confusing as in conversation and when explaining opaque type stuff to people I always just talk about things in terms of hidden types. Also the hidden types of opaques are very much not *concrete* in the same sense that a type without any generic parameters is concrete which is an unfortunate overlap in terminology.
I've tried to update comments to also stop referring to things as concrete opaque types but this is mostly best effort as it difficult to find all such cases amongst the massive amounts of uses of "concrete" or "hidden" across the whole compiler.
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