ada: Further cleanup in finalization machinery

The bodies of generic units are instantiated separately by GNAT at the end
of the processing of the compilation unit.  This requires the deferral of
the generation of cleanups and finalization actions in enclosing scopes,
except for instantiations in generic units where they are not generated.

The criterion used to detect this latter case is Inside_A_Generic, but this
global variable is not properly updated during the instantiation of generic
bodies, leading to problems with nested instantiations, so it is changed to
Expander_Active instead.  As a matter of fact, the exact same idiom is used
a few lines above to clear the Needs_Body variable.

gcc/ada/

	* sem_ch12.adb (Analyze_Package_Instantiation): Test Expander_Active
	to detect generic contexts for the generation of cleanup actions.

gcc/ada/sem_ch12.adb

@@ -4824,10 +4824,7 @@ package body Sem_Ch12 is
                      --  Cleanup actions are not generated within generic units
                      --  or in the formal part of generic units.
 
-                     if Inside_A_Generic
-                       or else Is_Generic_Unit (S)
-                       or else Ekind (S) = E_Void
-                     then
+                     if not Expander_Active then
                         exit;
 
                      --  For package scopes, cleanup actions are generated only