[flang] Retrieve rank before updating the pointer

[clementval]

Summary

The code is iterating on the rank of the pointer to set the bounds.
If the rank is retrieved after the pointer = target it does not
reflect the actual rank of the pointer.

This could happen in code like the following:

type t1
  integer :: a
end type

type(t), pointer :: p(:)
class(t), pointer :: q(:,:)
q(0:1,-2:2) => p(10:1:-1)

flang/runtime/pointer.cpp

dim.SetLowerBound(dim.Extent() == 0
             : GetInt64(lowerBounds.ZeroBasedIndexedElement<const char>(j),
                   boundElementBytes, terminator));
   }
 }
 
 void RTNAME(PointerAssociateRemapping)(Descriptor &pointer,
     const Descriptor &target, const Descriptor &bounds, const char *sourceFile,
     int sourceLine) {
+  int rank{pointer.rank()};
   pointer = target;
   pointer.raw().attribute = CFI_attribute_pointer;
-  int rank{pointer.rank()};
   Terminator terminator{sourceFile, sourceLine};
   SubscriptValue byteStride{/*captured from first dimension*/};
   std::size_t boundElementBytes{bounds.ElementBytes()};
   for (int j{0}; j < rank; ++j) {
     auto &dim{pointer.GetDimension(j)};
     dim.SetBounds(GetInt64(bounds.ZeroBasedIndexedElement<const char>(2 * j),
                       boundElementBytes, terminator),
         GetInt64(bounds.ZeroBasedIndexedElement<const char>(2 * j + 1),