Improve node type forward reference

This is a small code simplification.

In primnodes.h, we have

typedef struct IntoClause
{
...
/* materialized view's SELECT query */
Node *viewQuery ...;

with the comment

/* (Although it's actually Query*, we declare
* it as Node* to avoid a forward reference.)

But we can do this better by using an incomplete struct, like

struct Query *viewQuery ...;

That way, everything has the correct type and fewer casts are required.
This technique is already used elsewhere in node type definitions.

The second patch just removes some more unnecessary casts around
copyObject() that I found while working on this.