THIR patterns: Explicitly distinguish &pin from plain &/&mut

[Zalathar]

Currently, thir::PatKind::Deref is used for ordinary &/&mut patterns, and also for &pin const and &pin mut patterns under feature(pin_ergonomics). The only way to distinguish between them is by inspecting the Ty attached to the pattern node.

That's non-obvious, making it easy to miss, and is also a bit confusing to read when it does occur.

This PR therefore adds an explicit pin: hir::Pinnedness field to thir::PatKind::Deref, to explicitly distinguish pin-deref nodes from ordinary builtin-deref nodes.

(I'm not deeply familiar with the future of pin-patterns, so I'm not sure whether that information is best carried as a field or as a separate PatKind, but I think this approach is at least an improvement over the status quo.)

r? Nadrieril (or compiler)

[rustbot]

Some changes occurred in match checking

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Some changes occurred in exhaustiveness checking

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Some changes occurred in match lowering

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[rustbot]

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