we should report breaking change for x-ms-discriminator-value change

[qiaozha]

See Azure/azure-rest-api-specs#32483 (comment)
As you can see the definition for x-ms-discriminator-value it indicates the discriminator property's value on the wire layer, therefore, any changes related to x-ms-discriminator-value should be considered as an api layer breaking change.

A side note, the default x-ms-discriminator-value for any models that contains discriminator property is the model name itself. Which means if there's a model named A that doesn't have x-ms-discriminator-value before, and they newly added x-ms-discriminator-value and set it as B, it's also a breaking change.