migrating check_and_promote() and Expr::InitVal() to use IntrusivePtr's

[vpax]

check_and_promote() is a function declared in Val.h that currently takes a ValPtr argument and a const Type* argument (as well as a bool and an optional Location* argument). @timwoj flagged in his review of #1578 that it would be more tidy if the second argument were const TypePtr&. I looked into making that change (which I agree with) and found that doing so is fairly involved, since Expr::InitVal() itself takes a const Type* that it then passes to check_and_promote(), and Expr::InitVal() is called in a bunch of places.

This issue is meant to flag the general desirability of these changes, but at a low priority given it's likely a fair amount of low-level grunt work to actually implement.

[timwoj]

I looked at this briefly and there's honestly a ton more we could do here with migrating to use IntrusivePtr, even just in the Expr code. For example, all of these methods:

extern bool check_and_promote_exprs(ListExpr* elements, TypeList* types);
extern bool check_and_promote_args(ListExpr* args, const RecordType* types);
extern bool check_and_promote_exprs_to_type(ListExpr* elements, TypePtr type);

There's also two copies of the is_vector() method, one which takes a raw pointer and one which takes an IntrusivePtr. The former can be removed.

Doing all of the migration is honestly a project unto itself, and we should try to do all of it at once instead of piece-by-piece.