Rollup merge of #121892 - Zalathar:expr-kind-let, r=Nadrieril

The ordinary lowering of `thir::ExprKind::Let` is unreachable

After desugaring, `let` expressions should only appear inside `if` expressions or `match` guards, possibly nested within a let-chain. In both cases they are specifically handled by the lowerings of those expressions, so this case is currently unreachable.

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Context: https://rust-lang.zulipchat.com/#narrow/stream/182449-t-compiler.2Fhelp/topic/Lowering.20of.20.60thir.3A.3AExprKind.3A.3ALet.60.20is.20unreachable

My conclusions are partly based on the observation that stubbing out this match arm doesn't cause any test failures. So either this really is unreachable, or it can be reached in some obscure circumstances that our test suite doesn't cover.

If we end up needing this code (or something like it) for an implementation of https://github.com/rust-lang/rfcs/pull/3573, it should be easy enough to pull it back out of version control history.

I looked into having the `if`/`match` lowerings call back into `expr_into_dest`, but from what I can tell that won't work well, because there are extra scoping considerations that require some awareness of the enclosing if/match.

r? ```@Nadrieril```

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