[HLSL] Generate buffer subscript operators

In HLSL buffer types support array subscripting syntax for loads and
stores. This change fleshes out the subscript operators to become array
accesses on the underlying handle pointer. This will allow LLVM
optimization passes to optimize resource accesses the same way any other
memory access would be optimized.

Reviewed By: aaron.ballman

Differential Revision: https://reviews.llvm.org/D131268