Prohibit functions returning void as expressions

[lucivpav]

[lucivpav]

Note that using the output of a procedure as an expression currently results in an error that comes from LLVM. That's not optimal, but at least it does not compile.

[lucivpav]

procedure f(a : integer);
begin
  writeln(a);
end;

begin
  f(writeln(10));
end.

This program compiles and prints:

10
3

which is weird (undefined?).

Same behaviour for this program:

var a: integer;
begin
  a := writeln(10);
  writeln(a);
end.