Expressions passed by --macro should be parsed in macro context with -D macro

``` haxe
class ExprMacro {
public static function traceExpr(expr:haxe.macro.Expr):Void {
trace(expr);
}
}
```

```
$ haxe --macro "ExprMacro.traceExpr(macro 1)"
--macro:1: character 0 : { min : Int, max : Int, file : String } should be haxe.macro.Position
--macro:1: character 0 : Parameter should be a constant
```