[tcgc] roll back change we made that allowed any union of values to be an extensible enum

[iscai-msft]

In the last release, we decided to always generate a union of constants as an enum. However, after talking more with @lmazuel and seeing the confusion here, we've decided to only generate an enum from a union if the union is named

[tadelesh]

The root cause is not the anonymous, It is from the hierarchy. My fix will rollback the change for hierarchy. Details could be found in my PR's tests.