ada: Implement SPARK RM 6.9 (23)

ada: Implement SPARK RM 6.9 (23)

Implement the rule 23 for ghost code:

A user-defined primitive equality operation on a non-ghost record type shall
not be ghost, unless the record type has only limited views.
In addition, a user-defined primitive equality operation on a ghost record type
shall have a matching assertion level.

gcc/ada/ChangeLog:

	* ghost.adb (Check_Ghost_Equality_Op): New function for the
	implementation of the rule.
	* ghost.ads (Check_Ghost_Equality_Op): Likewise.
	* sem_ch6.adb (Valid_Operator_Definition): Add check for rule 23.