Rollup merge of #130526 - eholk:pin-reborrow, r=compiler-errors

Begin experimental support for pin reborrowing

This commit adds basic support for reborrowing `Pin` types in argument position. At the moment it only supports reborrowing `Pin<&mut T>` as `Pin<&mut T>` by inserting a call to `Pin::as_mut()`, and only in argument position (not as the receiver in a method call).

This PR makes the following example compile:

```rust
#![feature(pin_ergonomics)]

fn foo(_: Pin<&mut Foo>) {
}

fn bar(mut x: Pin<&mut Foo>) {
    foo(x);
    foo(x);
}
```

Previously, you would have had to write `bar` as:

```rust
fn bar(mut x: Pin<&mut Foo>) {
    foo(x.as_mut());
    foo(x);
}
```

Tracking:

- #130494

r? `@compiler-errors`

compiler/rustc_middle/src/ty/adjustment.rs

@@ -104,6 +104,9 @@ pub enum Adjust<'tcx> {
 
     /// Cast into a dyn* object.
     DynStar,
+
+    /// Take a pinned reference and reborrow as a `Pin<&mut T>` or `Pin<&T>`.
+    ReborrowPin(ty::Region<'tcx>, hir::Mutability),
 }
 
 /// An overloaded autoderef step, representing a `Deref(Mut)::deref(_mut)`