[flang][hlfir] Fixed finalization in hlfir.assign codegen.

When hlfir.assign is lowered into simple load/store,
we may still need to finalize the LHS.
The patch passes `needFinalization` to `genScalarAssignment`
for LHS of any derived type, so some `Destroy` calls
might be redundant. They can be removed later by propagating/deducing
IsFinalizable information about the LHS type.

Reviewed By: clementval

Differential Revision: https://reviews.llvm.org/D155664