There are three standards compliant ways of getting the tag field
for a union like GnmValue.

1. Via the subtype struct that the value has.  That only works (and
   is utterly pointless) when we already know the value type.

2. Via another union member that is a structure having one or more
   initial members matching the actual struct type.  The silly thing
   is that this rule only applies to structs; I guess that is an
   oversight, but a fat lot of good that does us now.  In practice
   this means we should use ->v_any.type for that.

3. Using the pointer-to-first-struct-member rule.

To simplify matters, define VALUE_IS_ARRAY and VALUE_IS_CELLRANGE
macros.