Rollup merge of #123864 - oli-obk:define_opaque_types3, r=compiler-errors

Remove a HACK by instead inferring opaque types during expected/formal type checking

I was wondering why I couldn't come up with a test that hits the code path of the argument check checking the types we inferred from the return type... Turns out we reject those attempts early during fudging.

I have absolutely no information for you as to what kind of type inference changes this may incur, but I think we should just land this out of two reasons:

* had I found the other place to use opaque type inference on before I added the hack, we'd be using that today and this PR would never have happened
* if it is possible to hit this path, it requires some god awful recursive RPIT logic that I doubt anyone would have written without actively trying to write obscure code

r? ``@ghost``

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