Stop handling opaque types in queries and leave it to typeck

[oli-obk]

Some cleanups now that all the lazy TAIT work has settled down.

We do not need to look into queries anymore, as all the opaque types get "equated away" early enough so that the trait solvers don't see opaque types unless they should treat them opaquely.

The last commit additionally flips the default on infcx.at(...).eq and friends, requiring you to opt-in to registering hidden types. We can probably do more cleanups here and possibly manage to eliminate the InferCtxt::defining_use_anchor field entirely by doing all processing at the define_opaque_types sites.

r? @lcnr