Rollup merge of #121928 - Zalathar:then-else-args, r=Nadrieril

Extract an arguments struct for `Builder::then_else_break`

Most of this method's arguments are usually or always forwarded as-is to recursive invocations.

Wrapping them in a dedicated struct allows us to document each struct field, and lets us use struct-update syntax to indicate which arguments are being modified when making a recursive call.

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While trying to understand the lowering of `if` expressions, I found it difficult to keep track of the half-dozen arguments passed through to every call to `then_else_break`. I tried switching over to an arguments struct, and I found that it really helps to make sense of what each argument does, and how each call is modifying the arguments.

I have some further ideas for how to streamline these recursive calls, but I've kept those out of this PR so that it's a pure refactoring with no behavioural changes.

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