[flang] Fix potential segfault in PointerAssociateRemapping

The bounds descriptor passed to the function is an array of [2, newRank]
size. Update the code so the rank is retrieved from the second dimension
and not the rank of the descriptor directly as it will be 2 in any case.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D144949