[flang] Fix bounds array creation for pointer remapping calls

`PointerAssociateRemapping` expect a descriptor holding
a newRank x 2 array of int64. The previous lowering was wrong.
Adapt the lowering to fit the expectation of the runtime.
Use the `bounds` to get the rank.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D142487