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

See https://github.com/Azure/azure-rest-api-specs/pull/32483#discussion_r1948548308
As you can see the definition for [x-ms-discriminator-value](https://github.com/Azure/autorest/tree/main/docs/extensions#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.

Start with the linked discussion and the x-ms-discriminator-value extension definition. Trace how the tool classifies discriminator changes between OpenAPI specifications. Done means changes to this value, including adding an explicit value where the model name was previously implicit, are reported as API-layer breaking changes.

索引モデルが issue の本文から書いたものです。