`adjust_upvar_deref` and friends are implemented so that they reuse
existing region so new region vars don't have to be generated.
Since now we don't have to generate region vars in `InferBorrowKind`,
creating a new capture info would be cheap and we can just use
`determine_capture_info`.
syn is updated so that let_else syntax can be used in fn with `#[instrument]` attribute.
`restrict_repr_packed_field_ref_capture` is changed to take place by value
since cloning is needed anyway.
52db6b9d02