[flang] Use rank from the allocate object when allocate with mold

[flang] Use rank from the allocate object when allocate with mold

The rank from the allocate object might be different from the rank
from the mold expression. Use the rank from the allocate object
when applying to mold so the bounds can be set correctly.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D143078