[flang] Use PointerAssociateLowerBounds when there is lower bounds

The current code was not taking provided lower bounds when the pointer
is polymorphic and was just calling PointerAssociate. This patch
updates the behavior and use PointerAssociateLowerBounds with the provided
lower bounds.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D143392