[flang] Make uninitialized allocatable components explicitly NULL() in structure constructors

When a structure constructor does not initialize an allocatable component,
ensure that the typed expression representation contains an explicit
NULL() for the component.  Expression semantics already copies default
initialized expressions for nonallocatable components into structure
constructors.  This change is expected to simplify lowering.

Differential Revision: https://reviews.llvm.org/D121162