Unify type comparisions for records.

For records, same_type(r1, r2) would not check if the fields'
attributes match as well. That seems like an oversight, and some
callers of same_type() did indeed add that check on their end. This
commit moves the check into same_type() itself. That generally doesn't
seem make any differences except for a couple of places validating
code, which we update a bit. That in turn leans to slightly different
(better?) error messages for a couple of test cases.

src/Expr.h

@@ -504,6 +504,11 @@ public:
 	ValPtr InitVal(const zeek::Type* t, ValPtr aggr) const override;
 	bool IsPure() const override;
 
+	void SetOp2(ExprPtr e)
+		{
+		op2 = e;
+		}
+
 protected:
 	bool TypeCheck(const AttributesPtr& attrs = nullptr);
 	bool TypeCheckArithmetics(TypeTag bt1, TypeTag bt2);