[flang][runtime] Invert component/element loops in assignment (#78341)

The general implementation of intrinsic assignment of derived types in
the runtime support library has a doubly-nested loop: an outer loop that
traverses the components and inner loops than traverse the array
elements. It's done this way to amortize the per-component overhead.
However, this turns out to be wrong when the program cares about the
order in which defined assignment subroutines are called; the Fortran
standard allows less latitude here than we need to invert the ordering
in this way when any component is itself an array. So invert the two
loops: traverse the array elements, and for each element, traverse its
components.