[flang] Ensure name resolution visits "=>NULL()" in entity-decl

Most modern Fortran programs declare procedure pointers with a
procedure-declaration-stmt, but it's also possible to declare one
with a type-declaration-stmt with a POINTER attribute.  In this
case, e.g. "real, external, pointer :: p => null()" the initializer
is required to be a null-init.  The parse tree traversal in name
resolution would visit the null-init if the symbol were an object
pointer only, leading to a crash in the case of a procedure pointer.

That explanation of the bug is longer than the fix.  In short,
ensure that a null-init in an entity-decl is visited for both
species of pointers.

Differential Revision: https://reviews.llvm.org/D129676