Auto merge of #75912 - scottmcm:manuallydrop-vs-forget, r=Mark-Simulacrum

Suggest `mem::forget` if `mem::ManuallyDrop::new` isn't used

I think this communicates the intent more idiomatically, and is shorter anyway.

Inspired because [it came up on URLO](https://users.rust-lang.org/t/validity-of-memory-area-after-std-forget/47730/7?u=scottmcm), and it turns out that std had done it too in one spot:
![image](https://user-images.githubusercontent.com/18526288/91203819-e19f2980-e6f2-11ea-9112-835f3b22ce05.png)

library/std/src/lazy.rs

@@ -293,7 +293,7 @@ impl<T> SyncOnceCell<T> {
 
         // Don't drop this `SyncOnceCell`. We just moved out one of the fields, but didn't set
         // the state to uninitialized.
-        mem::ManuallyDrop::new(self);
+        mem::forget(self);
         inner
     }