[flang][hlfir] Do not dereference unallocated entities in structure constructor.

Component-by-component assignment must be able to handle unallocated
allocatable values in structure constructor. F2018 7.5.10 p. 7 states
that the component must have unallocated status as a result of such
construction. The structure constructor temporary is initialized
such that all the allocatable components are unallocated, so we just
need to make sure not to do the component assignment if RHS is deallocated.

Depends on D152482  (the same LIT test is affected)

Reviewed By: jeanPerier, tblah

Differential Revision: https://reviews.llvm.org/D152493