[Ada] Mimic the C++ ABI when passing class-wide conversion actuals

This patch does not affect the behavior of Ada-only code but improves
consistency with the code generated by the C++ compiler.

2018-09-26  Javier Miranda  <miranda@adacore.com>

gcc/ada/

	* sem_res.adb (Resolve_Actuals): If the formal is a class-wide
	type conversion then do not skip resolving and expanding the
	conversion; required to displace the pointer to the object and
	reference the dispatch table associated with the target
	interface type.

From-SVN: r264624

gcc/ada/ChangeLog

+2018-09-26  Javier Miranda  <miranda@adacore.com>
+
+	* sem_res.adb (Resolve_Actuals): If the formal is a class-wide
+	type conversion then do not skip resolving and expanding the
+	conversion; required to displace the pointer to the object and
+	reference the dispatch table associated with the target
+	interface type.
+
 2018-09-26  Hristian Kirtchev  <kirtchev@adacore.com>
 
 	* libgnat/g-dynhta.adb (Prepend_Or_Replace): Update the number

gcc/ada/sem_res.adb

@@ -3809,6 +3809,7 @@ package body Sem_Res is
             if Ekind (F) /= E_In_Parameter
               and then Nkind (A) = N_Type_Conversion
               and then not Is_Class_Wide_Type (Etype (Expression (A)))
+              and then not Is_Interface (Etype (A))
             then
                if Ekind (F) = E_In_Out_Parameter
                  and then Is_Array_Type (Etype (F))