[flang] Deallocate local allocatable at end of their scopes (#67036)

Implement automatic deallocation of unsaved local alloctables when
reaching the end of their scope of block as described in Fortran 2018
9.7.3.2 point 2. and 3.

Uses genDeallocateIfAllocated used for intent(out) deallocation and the
"function context" already used for finalization at end of scope.