additionalProperties should not be allowed with x-ms-client-flatten: true extension

The following will not be allowed.
`StorageAccountProperties` will be flattened so we would not know where to put the additionalProperties.

```json5
{
"StorageAccount": {
"properties": {
"id": {
"type": "string"
},
"name": {
"type": "string"
},
"properties": {
"x-ms-client-flatten": true,
"$ref": "#/StorageAccountProperties"
}
}
},
"StorageAccountProperties": {
"additionalProperties": {
"type" : "object"
},
"properties": {
"p1": {
"type": "string"
},
"p2": {
"type": "string"
}
}
}
}
```

Moreover, it gets complicated if the `StorageAccount` object also had `"additionalProperties": {"type": "object"}`. Then the modeler will have a conflict between the top level additionalProperties and flattened additionalProperties.

So it is in the best interest to not allow model flattening along with additionalProperties.