[flang][runtime] Make ASSOCIATED() conform with standard

[klausler]

Summary

ASSOCIATED() must be false for zero-sized arrays, and the
strides on a dimension are irrelevant when the extent is unitary.

[jeanPerier]

Looks good

flang/runtime/pointer.cpp

bool RTNAME(PointerIsAssociatedWith)(
   if (pointer.raw().base_addr != target->raw().base_addr ||
       pointer.ElementBytes() != target->ElementBytes() ||
       rank != target->rank()) {
     return false;
   }
   for (int j{0}; j < rank; ++j) {
     const Dimension &pDim{pointer.GetDimension(j)};
     const Dimension &tDim{target->GetDimension(j)};
-    if (pDim.Extent() != tDim.Extent() ||
-        pDim.ByteStride() != tDim.ByteStride()) {
+    auto pExtent{pDim.Extent()};
+    if (pExtent == 0 || pExtent != tDim.Extent() ||
+        (pExtent != 1 && pDim.ByteStride() != tDim.ByteStride())) {
       return false;
     }
   }
   return true;
 }
 
 // TODO: PointerCheckLengthParameter, PointerAllocateSource
 
 } // extern "C"
 } // namespace Fortran::runtime