[flang] Catch and diagnose attempts to misuse NULL()

Some intrinsic functions can handle NULL() as an actual argument; most
can't.  Distinguish the two with a new ArgFlag facility in the intrinsic
procedure table.  Also transform some confusing Optionality codes that
were standing in for flags into ArgFlags.  Last, return false for a NULL()
pointer from the utility IsActuallyConstant(), ensuring that it can
still deal with a nested NULL() for components in structure constructors.

Differential Revision: https://reviews.llvm.org/D136893