LLVM: refactor array descriptor access

@czgdp18071 and I discussed how it could be done. It seems one idea that would work is this one:

• take all access to the descriptor and move it to a separate header file
• we could have a class and subclasses like RowMajor, ColumnMajor, ZOrder (https://en.wikipedia.org/wiki/Z-order_curve) that would implement such indexing and access to the descriptor.

We can start with just simple functions. Features that we have to expose:

• indexing
• size
• pointer to data

Whatever else is needed.