Deprecate and remove isdefined(a::Array, index::Int)

We shouldn't let people use `isdefined` with Arrays, since `isassigned` is perfectly good for Arrays and also does the right thing for AbstractArrays. Also, as @yuyichao points out, we will most likely want to separate the functions for #12447. The only non-trivial part here is the deprecation, which needs to happen in the C code since `isdefined` is not a generic function.
