Implement missing rule for covergroup expressions referencing constants

[MikePopoloski]

From the LRM:

Global and instance constants referenced from a covergroup_expression shall be members of the enclosing
class. The initializers for such instance constants shall appear before the referring covergroup constructor
call in the class constructor. These initializers shall not appear with the covergroup constructor call in the
body of any looping statement (see 12.7) or fork-join_none, either before or after.