Auto merge of #51964 - matthewjasper:unused-mut-mir-generation, r=nikomatsakis

[NLL] Fix various unused mut errors

Closes #51801
Closes #50897
Closes #51830
Closes #51904
cc #51918 - keeping this one open in case there are any more issues

This PR contains multiple changes. List of changes with examples of what they fix:

* Change mir generation so that the parameter variable doesn't get a name when a `ref` pattern is used as an argument
```rust
fn f(ref y: i32) {} // doesn't trigger lint
```
* Change mir generation so that by-move closure captures don't get first moved into a temporary.
```rust
let mut x = 0; // doesn't trigger lint
move || {
    x = 1;
};
```
* Treat generator upvars the same as closure upvars
```rust
let mut x = 0; // This mut is now necessary and is not linted against.
move || {
    x = 1;
    yield;
};
```

r? @nikomatsakis

0 files changed, 0 insertions, 0 deletions