No type inference for subslices in slice patterns

To reproduce, mouse hover over `_tail` in the following program:
```rust
fn main() {
    if let [_head, _tail @ ..] = &[1][..] {}
}
```

### Expected result
`&[i32]`

### Actual result
`{unknown}`

-------------

rust-analyzer version: 2020-06-08 (d696776)