ada: Fix spurious warning emissions

ada: Fix spurious warning emissions

Before this patch, warnings handled by `Sem_Warn.Check_References` were
erroneously emitted in some cases. Here is an example of a program that,
when compiled with the `-gnatwu` switch, triggered the bug:

    procedure Main is
       package T is
          A : Integer;
       end T;
    begin
       T.A := 7;
    end Main;

The following message was emitted:

   main.adb:3:07: warning: variable "A" is never read and never assigned [-gnatwu]

This patch mitigates the issue by restricting the cases in which
`Sem_Warn.Check_References` is called for package specifications.

Note that the recursive calls in `Sem_Warn.Check_References` can be used
to convince oneself that this patch does not remove legitimate warnings
for non-library-level package specifications.

gcc/ada/

	* sem_ch7.adb (Analyze_Package_Declaration): Restrict calls to
	`Sem_Warn.Check_References` and adjust comment accordingly.