Auto merge of #28748 - nikomatsakis:universal-mir, r=pnkfelix

I had to fix a few things. Notable changes:

1. I removed the MIR support for constants, instead falling back to the existing `ConstVal`. I still think we ought to reform how we handle constants, but it's not clear to me that the approach I was taking is correct, and anyway I think we ought to do it separately.
2. I adjusted how we handle bindings in matches: we now *declare* all the bindings up front, rather than doing it as we encounter them. This is not only simpler, since we don't have to check if a binding has already been declared, it avoids ICEs if any of the arms turn out to be unreachable.
3. I do MIR construction *after* `check_match`, because it detects various broken cases. I'd like for `check_match` to be subsumed by MIR construction, but we can do that as a separate PR (if indeed it makes sense).

I did a crater run and found no regressions in the wild: https://gist.github.com/nikomatsakis/0038f90e10c8ad00f2f8

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