Fix regression with dynamic array bounds

Fix regression with dynamic array bounds

Kévin discovered that commit ba005d32b0f ("Handle dynamic field
properties") regressed a test in the internal AdaCore test suite.

The problem here is that, when writing that patch, I did not consider
the case where an array type's bounds might come from a member of a
structure -- but where the array is not defined in the structure's
scope.

In this scenario the field-resolution logic would trip this condition:

  /* Defensive programming in case we see unusual DWARF.  */
  if (fi == nullptr)
    return nullptr;

This patch reworks this area, partly backing out that commit, and
fixes the problem.

In the new code, I chose to simply duplicate the field's location
information.  This isn't totally ideal, in that it might result in
multiple copies of a baton.  However, this seemed nicer than tracking
the DIE/field correspondence for every field in every CU -- my
thinking here is that this particular dynamic scenario is relatively
rare overall.  Also, if the baton cost does prove onerous, we could
intern the batons somewhere.

Regression tested on x86-64 Fedora 41.  I also tested this using the
AdaCore internal test suite.

Tested-By: Simon Marchi <simon.marchi@efficios.com>