uninit_assumed_init lint does not recognize trivial misuse

`uninit_assumed_init` lint (see #4272) correctly recognizes the following anti-pattern:

```rust
unsafe { let mut x = MaybeUninit::<&i32>::uninit().assume_init(); }
```

However, it does not recognize a slightly altered version of it:

```rust
    let mut x = MaybeUninit::<&i32>::uninit();
    let x = unsafe { x.assume_init() };
```

Clippy version: clippy 0.0.212 (c8e3cfb 2019-10-28)