TCGC should give a proper clientNamespace for anonymous models

[ArcturusZhang]

In some cases, the clientNamespace is empty in TCGC's output.
In typespec, everything should be defined in a context with a namespace therefore it is abnormal that TCGC is outputting something without a clientNamespace.

Anonymous models have generated names, and properly generated crossLanguageDefinitionId, therefore it is very possible that TCGC could give us so that the downstream generators do not have to make special logic around them.