Skip flattening if properties would clash

[danielrbradley]

Draft PR: #3016

Skip the flattening of nested properties indicated by x-ms-client-flatten if it would lead to overwriting a property, creating incorrect schema and SDKs. This case happens when inner and outer property have the same name, for a report on all occurrences see #3013.

This change is breaking and could therefore only be applied to v3 of the provider. The purpose of the PR is mainly to illustrate the SDK changes that would result.

Note: remember to remove exception added in #3007.