Adds a validator for the xmlFlattened trait that checks if the member's
target is a list that has a member with xmlName, and that xmlName
doesn't match the name of the xmlFlattened member. xmlFlattened causes
the xmlName of the list member to be ignored. This is called out in our
docs:
https://smithy.io/2.0/spec/protocol-traits.html#flattened-list-serialization,
but we were missing a validator for it. It is a warning because you might
mean to do this, i.e. if the list is used in multiple places.

I also added some suppressions to our protocol tests that are testing
the behavior for models that would trip up this validator.