[flang][runtime] Destroy nested allocatable components (#81117)

The runtime was currently only deallocating the direct allocatable
components, which caused leaks when there are allocatable components
nested in the direct components.

Update Destroy to recursively destroy components.

Also call Destroy from Assign to deallocate nested allocatable
components before doing the assignment as required by F2018 9.7.3.2
point 7.

This lack of deallocation was visible if the nested components had user
defined assignment "observing" the allocation state.