[flang][runtime] Accommodate change of type in assignment to allocatable (#141988)
When an assignment to a derived type allocatable requires
(re)allocation, its type may change to that of the right-hand side. The
code didn't update its derived type pointer, leading to the wrong type
being put into the descriptors created for elemental defined assignment
subroutine calls.
Fixes https://github.com/llvm/llvm-project/issues/141835.