SC_Obligations: rework the scopes querying API

Exactly one set of subprograms of interest is ever used: the set for
--subprograms command line arguments. Remove Is_Active's corresponding
argument and always use the set at Switches.Subps_Of_Interest instead.
This also opens the way to more efficient ways to check whether the
current SCO is covered by a scope that matches --subprograms.

Also rename Is_Active to avoid the confusion with the
Active_Scopes/Active_Scope_Ent components (which can contain scopes that
are *not* active according to --subprograms arguments).

Author: pmderodat

tools/gnatcov/annotations.adb

@@ -426,7 +426,7 @@ package body Annotations is
                for SCO of LI.SCOs.all loop
                   Traverse_SCO (ST, SCO);
                end loop;
-               if not Is_Active (ST, Subps_Of_Interest) then
+               if not In_Scope_Of_Interest (ST) then
                   return;
                end if;
             end if;

tools/gnatcov/coverage-source.adb

@@ -2003,7 +2003,7 @@ package body Coverage.Source is
       is
       begin
          Traverse_SCO (ST, SCO);
-         if Is_Active (ST, Subps_Of_Interest) then
+         if In_Scope_Of_Interest (ST) then
             Update_SCI (SCO, Tag, Process);
          end if;
       end Update_SCI_Wrapper;

tools/gnatcov/sc_obligations.adb

@@ -757,18 +757,16 @@ package body SC_Obligations is
       ST.Next_Scope_Ent := ST.It.Next (Scope_Ent);
    end Set_Active_Scope_Ent;
 
-   ---------------
-   -- Is_Active --
-   ---------------
+   --------------------------
+   -- In_Scope_Of_Interest --
+   --------------------------
 
-   function Is_Active
-     (ST                : Scope_Traversal_Type;
-      Subps_Of_Interest : Scope_Id_Set) return Boolean is
+   function In_Scope_Of_Interest (ST : Scope_Traversal_Type) return Boolean is
    begin
       return Subps_Of_Interest.Is_Empty
         or else not Scope_Id_Sets.Is_Empty
           (ST.Active_Scopes.Intersection (Subps_Of_Interest));
-   end Is_Active;
+   end In_Scope_Of_Interest;
 
    -----------------
    -- Add_Address --

tools/gnatcov/sc_obligations.ads

@@ -260,12 +260,11 @@ package SC_Obligations is
    --  Return the last SCO that was passed to Traverse_SCO, or No_SCO_Id if
    --  Traverse_SCO has not been called yet on ST.
 
-   function Is_Active
-     (ST                : Scope_Traversal_Type;
-      Subps_Of_Interest : Scope_Id_Set) return Boolean;
-   --  Return whether any of the scopes in Subps_Of_Interest is currently
-   --  active. Return True if Subps_Of_Interest is empty (i.e. consider all
-   --  subprograms of interest in that case).
+   function In_Scope_Of_Interest (ST : Scope_Traversal_Type) return Boolean;
+   --  Return whether at least one scope in Switches.Subps_Of_Interest contains
+   --  Last_SCO (ST). Note that this also returns True if
+   --  Switches.Subps_Of_Interest is empty (i.e. consider that all subprograms
+   --  are of interest in that case).
 
    No_Scope_Entity : constant Scope_Entity :=
      (From       => No_SCO_Id,