[clang][dataflow] Add two repros for non-convergence involving pointers in loops.

These are broken out from https://reviews.llvm.org/D156658, which it now seems obvious isn't the right way to solve the non-convergence.

Instead, my plan is to address the non-convergence through pointer value widening, but the exact way this should be implemented is TBD. In the meantime, I think there's value in getting these repros submitted to record the current undesirable behavior.

Reviewed By: ymandel, xazax.hun

Differential Revision: https://reviews.llvm.org/D158513