[flang] Fix codegen of subcomponents' indexing

Identify multidimensional array indices in subcomponents and
convert them from column-major to row-major ordering.

This fixes codegen for fircg.ext_array_coor, fircg.ext_embox and,
possibly, fircg.ext_rebox.

Fixes https://github.com/llvm/llvm-project/issues/62038

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D154214