[Ada] Restore defensive guard in checks for volatile actuals

[Ada] Restore defensive guard in checks for volatile actuals

When flagging names of volatile objects occurring in actual parameters
it is safer to guard against identifiers without entity. This is
redundant (because earlier in the resolution of actual parameters we
already guard against actuals with Any_Type), but perhaps such
identifiers will become allowed in constructs like:

   Subprogram_Call
     (Actual =>
        (declare
           X : Boolean := ...
             with Annotate (GNATprove, ...)));
                            ^^^^^^^^^

which include an identifier that does not denote any entity.

Code cleanup related to handling of volatile components; behaviour is
unaffected.

gcc/ada/

	* sem_res.adb (Flag_Effectively_Volatile_Objects): Restore
	redundant guard.