[Analyzer][VLASize] Support multi-dimensional arrays.

Summary:
Check the size constraints for every (variable) dimension of the array.
Try to compute array size by multiplying size for every dimension.

Reviewers: Szelethus, martong, baloghadamsoftware, gamesh411

Reviewed By: Szelethus, martong

Subscribers: rnkovacs, xazax.hun, baloghadamsoftware, szepet, a.sidorin, mikhail.ramalho, Szelethus, donat.nagy, dkrupp, gamesh411, Charusso, martong, ASDenysPetrov, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D77305