Auto merge of rust-lang#115166 - Urgau:invalid_ref_casting-invalid-unsafecell-usage, r=est31

Lint on invalid usage of `UnsafeCell::raw_get` in reference casting

This PR proposes to take into account `UnsafeCell::raw_get` method call for non-Freeze types for the `invalid_reference_casting` lint.

The goal of this is to catch those kind of invalid reference casting:
```rust
fn as_mut<T>(x: &T) -> &mut T {
    unsafe { &mut *std::cell::UnsafeCell::raw_get(x as *const _ as *const _) }
    //~^ ERROR casting `&T` to `&mut T` is undefined behavior
}
```

r? `@est31`

Author: bors

core/src/cell.rs

@@ -2165,6 +2165,7 @@ impl<T: ?Sized> UnsafeCell<T> {
     #[inline(always)]
     #[stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
     #[rustc_const_stable(feature = "unsafe_cell_raw_get", since = "1.56.0")]
+    #[rustc_diagnostic_item = "unsafe_cell_raw_get"]
     pub const fn raw_get(this: *const Self) -> *mut T {
         // We can just cast the pointer from `UnsafeCell<T>` to `T` because of
         // #[repr(transparent)]. This exploits std's special status, there is