[flang] Lower IO input with vector subscripts

This patch adds lowering for IO input with vector subscripts.
It defines a VectorSubscriptBox class that allow representing and working
with a lowered Designator containing vector subscripts while ensuring
all the subscripts expression are only lowered once.

This patch is part of the upstreaming effort from fir-dev branch.

Reviewed By: PeteSteinfeld

Differential Revision: https://reviews.llvm.org/D121806



Co-authored-by: Jean Perier <jperier@nvidia.com>
Co-authored-by: Eric Schweitz <eschweitz@nvidia.com>