This includes changing the feature name so that compilers with the experimental feature don’t accidentally pick up content that only works in the final version.
Resolves rdar://150065196.
It has indirect effects on the accessors, so it shouldn’t matter, but we can defensively redirect the query to the API counterpart anyway.
This was the last `InferredInABIAttr` attribute, so we can now remove all of the infrastructure involved in supporting attribute inference.
This commit compares the attributes on the decl inside the `@abi` attribute to those in the decl it’s attached to, diagnosing ABI-incompatible differences. It also rejects many attributes that don’t need to be specified in the `@abi` attribute, such as ObjC-ness, access control, or ABI-neutral traits like `@discardableResult`, so developers know to remove them.
