Fix Reference#dup of file private class

[makenowjust]

For instance:

private class Foo
end

Foo.new.dup

this code causes an error:

Error in foo.cr:4: instantiating 'Foo#dup()'

Foo.new.dup
        ^~~

in /.../src/reference.cr:40: expanding macro

    {% if @type.abstract? %}
    ^

in macro 'macro_XXX' /.../src/reference.cr:40, line 2:

   1.
>  2.       dup = Foo.allocate
   3.       dup.as(Void*).copy_from(self.as(Void*), instance_sizeof(Foo))
   4.       dup
   5.

This commit fixes it by wrapping {{@type}} expansion with #<loc:> pragma
of class defined location.

[asterite]

This is a breaking change

[asterite]

I closed this because this introduces breaking changes. I believe a better solution is to improve macros. Maybe macro interpolation {{...}} shouldn't work inside string literals, and then we could always emit loc pragmas.

A specific solution for dup might be to mention {%@type%} (just so that the compiler knows it's a macro def) and then do self.class.allocate

[makenowjust]

@asterite Macros in comment is disallowed, isn't it? If we can disallow macro in string literal, I think we can decide whether to output loc pragama on context, it is better.

[ysbaddaden]

Macros are allowed everywhere, including within comments.

[makenowjust]

But if loc pragma is emitted always, your comment will be so dirty.

[ysbaddaden]

Well, it renders properly in Crystal docs.

[asterite]

That's why I think the macros system should be improved, because when you pass nodes to them there's no easy way to preserve their location. And blindly outputting loc pragmas isn't the correct solution (for the yield inside macros this can be safely done because you usually yield a block of code and you wouldn't yield inside a string interpolation or other strange places)