[flang] Fix UBOUND() folding for constant arrays

Similarly to LBOUND in https://reviews.llvm.org/D123237, fix UBOUND() folding
for constant arrays (for both w/ and w/o DIM=): convert
GetConstantArrayLboundHelper into common helper class for both lower/upper
bounds.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D123520