Auto merge of #11981 - y21:eager_int_transmute, r=llogiq

new lint: `eager_transmute`

A small but still hopefully useful lint that looks for patterns such as `(x < 5).then_some(transmute(x))`.
This is almost certainly wrong because it evaluates the transmute eagerly and can lead to surprises such as the check being completely removed and always evaluating to `Some` no matter what `x` is (it is UB after all when the integer is not a valid bitpattern for the transmuted-to type). [Example](https://godbolt.org/z/xoY34fPzh).
The user most likely meant to use `then` instead.

I can't remember where I saw this but this is inspired by a real bug that happened in practice.

This could probably be a correctness lint?

changelog: new lint: [`eager_int_transmute`]

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