[flang] Convert assertion to a TODO

The original assertion is not necessarily correct since the shape
argument may involve a slice of an array (an expression) and not a whole
vector with constant length. In the presence of a slice operation, the
size must be computed (left as a TODO for now).

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

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D128894



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