[Ada] Subprogram_Variant in ignored ghost code

gcc/ada/

	* exp_ch6.adb (Expand_Call_Helper): Do not call
	Check_Subprogram_Variant if the subprogram is an ignored ghost
	entity. Otherwise the compiler crashes (in debug builds) or
	gives strange error messages (in production builds).

gcc/ada/exp_ch6.adb

@@ -4392,6 +4392,7 @@ package body Exp_Ch6 is
       --  the current subprogram is called.
 
       if Is_Subprogram (Subp)
+        and then not Is_Ignored_Ghost_Entity (Subp)
         and then Same_Or_Aliased_Subprograms (Subp, Current_Scope)
       then
          Check_Subprogram_Variant;