[flang][hlfir] Use complete form of DesignateOp for im/re array designators.

With this change, the lowering provides full slice triples for
designators like ...%array_comp%im/re, so that the codegen
does not have to figure it out for the array_comp.
Basically, ...%array_comp%im/re is lowered the same way
as ...%array_comp(:,:,...)%im/re.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D150842