Make OnceCell<T> transparent to dropck

See the failed build in

https://github.com/rust-lang/rust/pull/75555#issuecomment-675016718

for an example where we need this in real life

library/core/tests/lazy.rs

@@ -122,3 +122,12 @@ fn reentrant_init() {
     });
     eprintln!("use after free: {:?}", dangling_ref.get().unwrap());
 }
+
+#[test]
+fn dropck() {
+    let cell = OnceCell::new();
+    {
+        let s = String::new();
+        cell.set(&s).unwrap();
+    }
+}

library/std/src/lazy.rs

@@ -386,9 +386,10 @@ impl<T> SyncOnceCell<T> {
     }
 }
 
-impl<T> Drop for SyncOnceCell<T> {
+unsafe impl<#[may_dangle] T> Drop for SyncOnceCell<T> {
     fn drop(&mut self) {
-        // Safety: The cell is being dropped, so it can't be accessed again
+        // Safety: The cell is being dropped, so it can't be accessed again.
+        // We also don't touch the `T`, which validates our usage of #[may_dangle].
         unsafe { self.take_inner() };
     }
 }
@@ -845,4 +846,13 @@ mod tests {
             assert_eq!(msg, MSG);
         }
     }
+
+    #[test]
+    fn dropck() {
+        let cell = SyncOnceCell::new();
+        {
+            let s = String::new();
+            cell.set(&s).unwrap();
+        }
+    }
 }