ada: Tidy up code dealing with visibility in instances of formal packages

ada: Tidy up code dealing with visibility in instances of formal packages

The actuals for parameters of formal packages are visible in the instance
if the formal packages either are declared with a box or contain defaulted
parameters.  This is essentially implemented in Check_Generic_Actuals and
Restore_Private_Views, with the help of the Is_Visible_Formal flag.

This documents more prominently the processing and streamlines it, as well
as removes a couple of questionable calls to Set_Is_Hidden, thus plugging
a couple more loopholes in the implementation.

gcc/ada/ChangeLog:

	PR ada/122161
	* sem_ch12.adb: Improve the description of Check_Generic_Actuals,
	Restore_Private_Views and Switch_View, and alphabetize them.
	(Check_Generic_Actuals): Make actuals visible only if needed.
	(Install_Formal_Packages): Pass exact Is_Formal_Box parameter and
	remove call to Set_Is_Hidden.
	(Instantiate_Formal_Package): Use Next_Non_Pragma consistently in
	the loop computing the visibility of actuals.
	(Analyze_Package_Instantiation): Use named second parameter in the
	call to Restore_Private_Views.
	(Analyze_Subprogram_Instantiation): Likewise.
	(Instantiate_Package_Body): Likewise and for Check_Generic_Actuals.
	(Instantiate_Subprogram_Body): Likewise.
	(Restore_Env): Replace call to Restore_Private_Views.
	(Restore_Private_Views): Remove default value for second parameter
	and test of presence for first parameter.  Consistently clear the
	Is_Potentially_Use_Visible flag when setting the Is_Hidden flag.
	Remove call to Set_Is_Hidden for a formal package of the enclosing
	generic unit, if any.