Eliminate cast from &T to &mut T

Summary:
This code is 100% UB and no longer compiles in Rust 1.73.

```
error: casting `&T` to `&mut T` is undefined behavior, even if the reference is unused, consider instead using an `UnsafeCell`
   --> fbcode/hermetic_infra/reverie/experimental/reverie-sabre/src/thread.rs:652:32
    |
652 |             let slot_map_mut = &mut *((&*SLOT_MAP as *const _) as *mut _);
    |                                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
    |
    = note: `#[deny(invalid_reference_casting)]` on by default
```

Reviewed By: JakobDegen

Differential Revision: D50488164

fbshipit-source-id: ea649d97f47ccc329a44650aff5dc467ed12b48c

experimental/reverie-sabre/src/thread.rs

@@ -595,6 +595,7 @@ impl<E: EventSink> Thread<E> {
 mod tests {
     use std::collections::HashSet;
     use std::mem;
+    use std::ptr;
     use std::sync::atomic::fence;
     use std::sync::atomic::AtomicBool;
     use std::sync::Arc;
@@ -638,7 +639,6 @@ mod tests {
         Thread::<TestEventSink>::current()
     }
 
-    #[allow(clippy::cast_ref_to_mut)]
     pub fn run_test_in_new_thread<T>(t: T)
     where
         T: 'static + Send + FnOnce(),
@@ -649,7 +649,8 @@ mod tests {
         // test run. This is safe because each test is running inside a single
         // mutex, so only one test will be accessing the static variable at once
         unsafe {
-            let slot_map_mut = &mut *((&*SLOT_MAP as *const _) as *mut _);
+            // FIXME(JakobDegen): This is UB
+            let slot_map_mut = ptr::addr_of!(*SLOT_MAP).cast_mut();
 
             *slot_map_mut = SlotMap::<ThreadRepr>::new();
         }