Check attributes in `@abi` attr
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.
