An enum constant isn't a macro; it can't be #undef-ined.  This
distinction will matter in a future commit, so create a separate flag
for elements like this.  I suspect this would also apply to other types
of elements, such as struct members, should we eventually need to deal
with those.

Previously, it was considered a macro.  But the previous commit created
a new flag for elements like this, which this commit now changes to use.