mir_build: Don't use a mixture of THIR pattern kinds for pin-patterns

[Zalathar]

While looking for improvements to match-lowering, I had been trying to figure out why pin-patterns sometimes use PatKind::Deref and sometimes use PatKind::DerefPattern, which seemed confusing to me. In particular, both pattern kinds are being used with an inner type of & or &mut, which is very surprising.

The conclusion I came to was that DerefPattern (normally associated with calls to Deref/DerefMut) was only being used for implicit pin-patterns produced by match-ergonomics adjustment, whereas Deref was being used for explicit pin-patterns.

The inconsistency seems like a mistake to me, so this PR removes all uses of DerefPattern for pin-patterns, and consistently uses PatKind::Deref for pin-patterns instead. I'm not entirely happy with that outcome, because I think pin-patterns should probably have their own thir::PatKind variant, but this change will at least make that easier to achieve in a later PR.

r? Nadrieril

[rustbot]

Some changes occurred in match checking

cc @Nadrieril

Some changes occurred in match lowering

cc @Nadrieril

[rustbot]

Nadrieril is not on the review rotation at the moment.
They may take a while to respond.