[Feature Request] Get <input> in autocomplete-suggestions using getInput()

### Existing Component
Yes

### Component Name
Autocomplete

### Description
#### Get `` in autocomplete-suggestions using `getInput()` method

Autocomplete-suggestions.vue component contains a reference, which allows to get input value from parent component.

```javascript
// element-ui/packages/autocomplete/src/autocomplete-suggestions.vue

this.referenceElm = this.$parent.$refs.input.$refs.input;
```

**But the component `el-input` already has a `getInput()` method, why we can't just use it instead of `$refs`?**

```javascript
// element-ui/packages/input/src/input.vue

methods: {
getInput() {
return this.$refs.input || this.$refs.textarea;
}
}
```

The issue: when we make a wrapper for `el-input`, the `autocomplete-suggestions` component gets incorrect `$refs`, which can be avoided by using a `getInput()` method.

Exemple: https://gist.github.com/martdn/1da5addd6a0f55c235c97d4c4e7053bd

**Steps to reproduce**
1. Wrap `el-input` in a component, e.g. `v-input`
2. Create a wrapper for `autocomplete` and change `el-input` stock component to `v-input` custom component in the template https://gist.github.com/martdn/1da5addd6a0f55c235c97d4c4e7053bd#file-autocomplete-vue
3. In this case `autocomplete-suggestions` addresses the `v-input` component by `$refs` and throws an error