[flang] Accommodate arrays with a zero-extent dimension in location folding

The index incrementation code used for FINDLOC, MAXLOC, and MINLOC folding
would crash if the array had a zero extent on the dimension selected with
a DIM= argument since the subscript passed to IncrementSubscripts would
have a value less than the lower bound.  Fix, and add tests.

Differential Revision: https://reviews.llvm.org/D121125