Add conditional attribute removal annotation

Since 5.0 there is forced attribute removal annotation called `RemoveAttributeInstances ` which can be used to always remove an attribute. As we won't be easily able to run linker in the mode where it removes all unused attribute we could extend this model with conditional attribute removal which would remove the attribute unless there is type-ref to its type.

This is alternative proposal to #1378

This can be useful for widely used core attributes which are needed only in specific features (e.g. serialization). A few examples would be

ObsoleteAttribute
CompileGeneratedAttribute
Serializable* attributes

/cc @eerhardt @vitek-karas