merge `BorrowKind::Unique` into `BorrowKind::Mut`

[lcnr]

We currently have a separate mir::BorrowKind for the mutable borrows for closure upvars.

BorrowKind::Unique is an ordinary mutable borrow, requiring its target to be invariant, needing unique access, except that the target is not necessarily required to be marked as mut by the user.

We should merge this into BorrowKind::Mut, resulting in the following setup:

enum BorrowKind {
    Shared,
    Shallow,
    Mut { kind: MutBorrowKind },
}

enum MutBorrowKind {
    Default,
    TwoPhaseBorrow,
    ClosureCapture,
}

It is quite worrying to me that fn BorrowKind::mutability simply returns Mutability::Not for BorrowKind::Unique

rust/compiler/rustc_middle/src/mir/mod.rs

Lines 2030 to 2035 in 99ff5af

	pub fn mutability(&self) -> Mutability {
	match *self {
	BorrowKind::Shared | BorrowKind::Shallow | BorrowKind::Unique => Mutability::Not,
	BorrowKind::Mut { .. } => Mutability::Mut,
	}
	}

This already caused a bug in #112056 where we used PlaceContext::NonMutatingUse for unique borrows, which resulted in us getting the wrong variance, cc #112070.

[RalfJung]

Can BorrowKind::Unique be used to perform actual mutation of a let (not let mut) bound local, or can it only be used for mutation through such a local (e.g. a reference stored there)?

[lcnr]

Can BorrowKind::Unique be used to perform actual mutation of a let (not let mut) bound local, or can it only be used for mutation through such a local (e.g. a reference stored there)?

afaict it is only used to deal with locals whose type is &mut x, so BorrowKind::Unique only allows mutation through such a local