Given the primary clause name and the field name, returns the "unique" clause name that should be used when
constructing terminal clauses for this clause name. Uniqueness is calculated per user; a name could be unique
for one user since he only has limited view of fields, but for another user it could be non-unique.
user - the user performing the search
primaryName - the primary name of a clause, e.g. cf or project
fieldName - the name of the field associated to the clause, e.g. My Custom Field or project
the clause name which should be used in construction of terminal clauses, to guarantee that this clause
refers only to the one specific field.