Subtypes of immutable types -- linting their field assignments

[tonyhffong]

Complex and AbstractArray are abstract immutable types.

However, Complex{Float64} is an concrete immutable type, but many subtypes of AbstractArray are mutable. There's no way for lint to tell if a generic function defined on the abstract type is trying to mutate an immutable subtype, or not.

While trying to catch setting complex field, there are now lots of false positives in linting functions on AbstractArrays.