Auto merge of #46236 - davidtwco:issue-46023, r=arielb1

MIR-borrowck: immutable unique closure upvars can be mutated

Fixes #46023 and #46160 (see [this comment](https://github.com/rust-lang/rust/pull/46236#issuecomment-347204874)).

diff --cc src/librustc/ich/impls_mir.rs
Simple merge

diff --cc src/librustc/mir/mod.rs
Simple merge

diff --cc src/librustc_mir/build/mod.rs
Simple merge