[flang] Support lowering of parent component in strecture constructor

Reuse the previous record assignment code. Use gen(expr) to generate
the fir::ExtendedValue of parent component recursively and convert the
structure to a fir.ref<T> type, where T is the record type of parent
component. Then, fir::factory::genRecordAssignment can be reused to
assign the parent component to the type-casted structure.

Reviewed By: jeanPerier

Differential Revision: https://reviews.llvm.org/D136530