[flang] simplify pointer assignments (#168732)
Pointer assignment lowering was done in different ways depending on
contexts and types, sometimes still using runtime calls when this is not
needed and the complexity of doing this inline is very limited (the
pointer and target descriptors were already prepared inline, the runtime
is just doing the descriptor assignment and ensuring the pointer
descriptor keep its pointer flag).
Slightly extent the inline version that was used for Forall and use it
for all cases.
When lowering without HLFIR is removed, this will allow removing more
code.
