[MatStepper] FormControlLike is not compatible with FormGroup

#### What is the expected behavior?
The stepControl attribute of the CdkStep/MatStep classes should be castable to/assignable from a FormGroup or FormArray.

#### What is the current behavior?
`Conversion of type 'FormControlLike' to type 'FormGroup' may be a mistake because neither type sufficiently overlaps with the other. If this was intentional, convert the expression to 'unknown' first.
Property 'controls' is missing in type 'FormControlLike'.`
Happens during compilation when trying to cast FormControlLike to a FormGroup.

```Type FormGroup is not assignable to type FormControlLike```
Is the warning shown in HTML files when trying to assign a FormGroup to the stepControl input of a MatStep.

#### What are the steps to reproduce?
Create a MatStepper with a FormGroup assigned to a stepControl of one of the MatSteps. To reproduce the compilation issue, try to cast the stepControl of one step into a FormGroup within a TypeScript function.

#### Which versions of Angular, Material, OS, TypeScript, browsers are affected?
Material Version 7.3.7

Reproduisez le problème de compilation avec Angular Material 7.3.7 et TypeScript en utilisant un MatStepper dont le MatStep stepControl est un FormGroup. Inspectez d’abord le typage de CdkStep/MatStep stepControl ; c’est terminé lorsque les affectations et les casts de FormGroup et FormArray se compilent sans les erreurs signalées.

Rédigé par le modèle d'indexation à partir du texte de l'issue.