bug(mat-autocomplete): missing selection indicator (highlight + check icon) when using mat-autocomplete with mat-optgroup

### Is this a regression?

- [ ] Yes, this behavior used to work in the previous version

### The previous version in which this bug was not present was

14

### Description

v15 introduced an indicator of the currently selected item:
However, this indicator is missing when, instead of flat mat-options, we're using mat-optgroup with mat-options:

### Reproduction

The repro is even in the official documentation of any version higher than v14:
https://v15.material.angular.io/components/autocomplete/examples

### Expected Behavior

We should see the indicator of a selected item even when we're using mat-optgroup

### Actual Behavior

Missing selection indicator when using mat-autocomplete with mat-optgroup

### Environment

- Angular: v15+
- CDK/Material: v15+

从 issue 中链接的 autocomplete 示例开始，比较扁平 mat-options 情况和 mat-optgroup 情况。在 Angular/Material v15+ 上重现缺失的高亮和勾选图标，然后验证分组选项也会显示所选项目指示器。

由索引模型根据 Issue 内容生成。