additionalProperties: true does not work on a model being flattened

[ArcturusZhang]

This happens to both swagger and typespec input, but since typespec input we should not turn the client-flatten on, this theoretically only affects swagger input libraries.

If we have additionalProperties: true on a model, but it is marked as x-ms-client-flatten: true on the other model, for example:

"VirtualMachine": {
"properties": {
"$ref": "#/definition/VirtualMachineProperties",
"x-ms-client-flatten": true
}
},
"VirtualMachineProperties": {
"additionalProperties": true
}

In the generated code, the additionalProperties flag in VirtualMachineProperties is not honored at all.