Auto merge of #112070 - lcnr:disjoint-closure-capture-ub, r=oli-obk

change `BorrowKind::Unique` to be a mutating `PlaceContext`

fixes #112056

I believe that `BorrowKind::Unique` is a footgun in general, so I added a FIXME and opened rust-lang/rust#112072. This is a bit too involved for this PR though.

Author: bors