Only declaration required as make_wrapped_policy template params

[SergeyKopienko]

In this PR we doing the next things:

1. Only declaration required as make_wrapped_policy template param
2. For aligning we declare these things always as struct.

The goal - to align these structures: their body isn't required.
And we have a lot of the same structures without body.
For example,

template <typename Name>
class ExclusiveScan1;
template <typename Name>
class ExclusiveScan2;

and so far and so on.

[SergeyKopienko]

@rarutyun, how do you think, does it make sense to apply this PR ?

[danhoeflinger]

I don't believe there is any downside to making this change to be declaration only, since these types are merely used as indicator types to make unique and identifiable policy names. However, I am not 100% sure that making the change to declaration only is without negative consequences in all compilers / environments.

I don't see much benefit to the change, as it only removes a handful of braces / lines. I suppose it may prevent someone from doing something with the type for some other purpose beyond how it is currently used as a way to make a unique policy name.

Alignment is the most interesting part to me, but I think the most important part of an alignment would be the name, because the name is effectively the only purpose of these types. I've pointed out a few outliers and places we can improve.

I don't see a benefit to aligning class vs struct, but I don't object to it either.

[danhoeflinger]

After offline discussion, I realize I've missed that we have many such instances of declaration only objects used in this fashion throughout oneDPL (I think I've written some).
With this in mind, I think this is safe to do. I wont insist on naming changes, because that would make this PR much larger than its original intentions, but I do think it could be a benefit.

I see no problem with this PR merging.

[MikeDvorskiy]

LGTM