[Ada] Spurious use_type clause warning

This patch corrects an error in the compiler whereby a spurious
redundant use_type_clause warning gets issued when the clause appears in
the context_clause of a package preceding a with_clause for a package
with an identical use_clause in its specification.

gcc/ada/

	* einfo.ads: Modify documentation for In_Use flag to include
	scope stack manipulation.
	* sem_ch8.adb (Use_One_Type): Add condition to return when
	attempting to detect redundant use_type_clauses in child units
	in certain cases.

gcc/ada/einfo.ads

@@ -2309,6 +2309,10 @@ package Einfo is
 --       the corresponding entity. Reset at end of corresponding declarative
 --       part. The flag on a type is also used to determine the visibility of
 --       the primitive operators of the type.
+--
+--       Note that manipulation of scopes on the scope stack will also cause
+--       the flag to be set/unset since the setting of scopes affects
+--       visibility.
 
 --    Is_Abstract_Subprogram
 --       Defined in all subprograms and entries. Set for abstract subprograms.

gcc/ada/sem_ch8.adb

@@ -10758,15 +10758,26 @@ package body Sem_Ch8 is
                      return;
                   end if;
 
-                  --  There is a redundant use_type_clause in a child unit.
-                  --  Determine which of the units is more deeply nested. If a
+                  --  If there is a redundant use_type_clause in a child unit
+                  --  determine which of the units is more deeply nested. If a
                   --  unit is a package instance, retrieve the entity and its
                   --  scope from the instance spec.
 
                   Ent1 := Entity_Of_Unit (Unit1);
                   Ent2 := Entity_Of_Unit (Unit2);
 
-                  if Scope (Ent2) = Standard_Standard then
+                  --  When the scope of both units' entities are
+                  --  Standard_Standard then neither Unit1 or Unit2 are child
+                  --  units - so return in that case.
+
+                  if Scope (Ent1) = Standard_Standard
+                    and then Scope (Ent2) = Standard_Standard
+                  then
+                     return;
+
+                  --  Otherwise, determine if one of the units is not a child
+
+                  elsif Scope (Ent2) = Standard_Standard then
                      Error_Msg_Sloc := Sloc (Clause2);
                      Err_No := Clause1;