Rollup merge of rust-lang#52149 - willmo:transparent-atomics, r=cramertj

Add #[repr(transparent)] to Atomic* types

This allows them to be used in `#[repr(C)]` structs without warnings. Since rust-lang/rfcs#1649 and rust-lang#35603 they are already documented to have "the same in-memory representation as" their corresponding primitive types. This just makes that explicit.

This was briefly part of rust-lang#51395, but was controversial and therefore dropped. But it turns out that it's essentially already documented (which I had forgotten).

src/libcore/sync/atomic.rs

@@ -124,6 +124,7 @@ pub fn spin_loop_hint() {
 /// [`bool`]: ../../../std/primitive.bool.html
 #[cfg(target_has_atomic = "8")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[repr(transparent)]
 pub struct AtomicBool {
     v: UnsafeCell<u8>,
 }
@@ -147,6 +148,7 @@ unsafe impl Sync for AtomicBool {}
 /// This type has the same in-memory representation as a `*mut T`.
 #[cfg(target_has_atomic = "ptr")]
 #[stable(feature = "rust1", since = "1.0.0")]
+#[repr(transparent)]
 pub struct AtomicPtr<T> {
     p: UnsafeCell<*mut T>,
 }
@@ -976,6 +978,7 @@ macro_rules! atomic_int {
         ///
         /// [module-level documentation]: index.html
         #[$stable]
+        #[repr(transparent)]
         pub struct $atomic_type {
             v: UnsafeCell<$int_type>,
         }