Auto merge of #6181 - cgm616:undropped-manually-drops, r=flip1995

Add new lint for undropped ManuallyDrop values

Adds a new lint for the following code:

```rust
struct S;

impl Drop for S {
    fn drop(&mut self) {
        println!("drip drop");
    }
}

fn main() {
    // This will not drop the `S`!!!
    drop(std::mem::ManuallyDrop::new(S));
    unsafe {
        // This will.
        std::mem::ManuallyDrop::drop(&mut std::mem::ManuallyDrop::new(S));
    }
}
```

The inner value of a `ManuallyDrop` will not be dropped unless the proper, unsafe drop function is called on it. This lint makes sure that a user does not accidently use the wrong function and forget to drop a `ManuallyDrop` value.

Fixes #5581.

---

*Please keep the line below*
changelog: none

tests/ui/undropped_manually_drops.rs

@@ -0,0 +1,26 @@
+#![warn(clippy::undropped_manually_drops)]
+
+struct S;
+
+fn main() {
+    let f = std::mem::drop;
+    let g = std::mem::ManuallyDrop::drop;
+    let mut manual1 = std::mem::ManuallyDrop::new(S);
+    let mut manual2 = std::mem::ManuallyDrop::new(S);
+    let mut manual3 = std::mem::ManuallyDrop::new(S);
+    let mut manual4 = std::mem::ManuallyDrop::new(S);
+
+    // These lines will not drop `S` and should be linted
+    drop(std::mem::ManuallyDrop::new(S));
+    drop(manual1);
+
+    // FIXME: this line is not linted, though it should be
+    f(manual2);
+
+    // These lines will drop `S` and should be okay.
+    unsafe {
+        std::mem::ManuallyDrop::drop(&mut std::mem::ManuallyDrop::new(S));
+        std::mem::ManuallyDrop::drop(&mut manual3);
+        g(&mut manual4);
+    }
+}

tests/ui/undropped_manually_drops.stderr

@@ -0,0 +1,19 @@
+error: the inner value of this ManuallyDrop will not be dropped
+  --> $DIR/undropped_manually_drops.rs:14:5
+   |
+LL |     drop(std::mem::ManuallyDrop::new(S));
+   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+   |
+   = note: `-D clippy::undropped-manually-drops` implied by `-D warnings`
+   = help: to drop a `ManuallyDrop<T>`, use std::mem::ManuallyDrop::drop
+
+error: the inner value of this ManuallyDrop will not be dropped
+  --> $DIR/undropped_manually_drops.rs:15:5
+   |
+LL |     drop(manual1);
+   |     ^^^^^^^^^^^^^
+   |
+   = help: to drop a `ManuallyDrop<T>`, use std::mem::ManuallyDrop::drop
+
+error: aborting due to 2 previous errors
+