astgen: generate type specific addNext, remove astNextNull

Generate type specific static overloads of Ast<Node>::addNext, which
return the correct sub-type of the 'this' they were invoked on.

Also remove AstNode::addNextNull, which is now only used in the parser,
implement in verilog.y directly as a template function.

src/V3VariableOrder.cpp

@@ -186,7 +186,7 @@ class VariableOrder final {
             for (; it != varps.cend(); ++it) firstp->addNext(*it);
             if (AstNode* const stmtsp = modp->stmtsp()) {
                 stmtsp->unlinkFrBackWithNext();
-                firstp->addNext(stmtsp);
+                AstNode::addNext<AstNode, AstNode>(firstp, stmtsp);
             }
             modp->addStmtp(firstp);
         }