[Ada] Fix spurious -Wuninitialized warnings for small records

[Ada] Fix spurious -Wuninitialized warnings for small records

This change is aimed at getting rid of spurious -Wuninitialized warnings
issued for small records passed by copy and containing default values
for some of their components.

The source of the problem is that the _Init parameter of the
initialization routine is declared as an in/out parameter, so the
uninitialized object is passed by copy to it and this can be flagged by
-Wuninitialized.

That's why the mode of the parameter is changed to out, except for the
cases where information really needs to be passed in: unconstrained
array types, protected and task types.

For the following record type Rec!

 type Rec is record
    B : Boolean := True;
  end record;

the initialization routine must now be:

      procedure r__recIP (_init : out r__rec1) is
      begin
         _init.b := true;
         return;
      end r__recIP;

2018-10-09  Eric Botcazou  <ebotcazou@adacore.com>

gcc/ada/

	* exp_ch3.adb (Is_Null_Statement_List): New predicate.
	(Build_Array_Init_Proc): Use it to find out whether the
	initialization procedure Is_Null_Init_Proc; if so, set
	Warnings_Off on the parameter.
	(Build_Init_Procedure): Likewise.
	(Init_Formals): Use an in/out first parameter only for
	unconstrained arrays and for records either containing or built
	for proteced types or task types; use an out parameter in all
	the other cases.
	* fe.h (Is_Init_Proc): Declare.
	* gcc-interface/decl.c (type_requires_init_of_formal): Do not
	return true for a discriminant in an unchecked union.
	(gnat_to_gnu_param): Do not create a PARM_DECL for the Out
	parameter of an initialization procedure.

From-SVN: r264984