[flang] enable as_expr codegen from array without FortranVariabeInterface

The defining op of HLFIR variables is expected to be visible
in most cases, but HLFIR codegen won't rely on it from a correctness
point of view.

This patch allows building a fir.shape from an hlfir::Entity does not
have a visible FortranVariabeInterface defining op.

Differential Revision: https://reviews.llvm.org/D140099