a11y: mat-select components can create aria-labelledby attributes with invalid values

[chrisboaks]

Bug, feature request, or proposal:

a11y bug: mat-select components can create aria-labelledby attributes with invalid values.

What is the expected behavior?

mat-select aria-labelledby values are always valid.

What is the current behavior?

mat-select aria-labelledby values are not always valid.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-vkhysf-pmuokg?file=app/select-value-binding-example.html

1. Inspect the mat-select tag on the above page
2. Note the value in the aria-labelledby attribute ("mat-form-field-label-1")
3. Note that there are no elements on the page with that id.

What is the use-case or motivation for changing an existing behavior?

Achieving GAR4 in my app.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

Angular + Material: HEAD
OS/TS/Browsers: all

Is there anything else we should know?

I believe the issue was introduced in this commit: e349fe4

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