Fix -Wc++11-narrowing warnings

[AlexeySachkov]

Newer versions of clang make it error by default, which affect clang-based implementations (like DPCPP).

[AlexeySachkov]

Note for reviewers: the main change is in tests/common/type_list.h, the rest is purely mechanical and caused by the helper refactoring (function -> struct static member function).

It is possible to preserve the helper in form of a function, by doing something like:

template <typename T>
inline constexpr auto get_init_value_helper(int x) {
  return T{ static_cast<typename get_element_type<T>::type>(x) };
}

Where get_element_type is a new (or existing) type trait to get scalar element type out of vec and marray. This way the change would be less invasive. However, I do like the new name better, because with the old name one might think that function is returning some kind of helper instead of an init value. Let me know about the preference, I can rework the PR accordingly.

[keryell]

What is the value of having struct instead of generic functions like

Suggested change

	user_def_types::init_value_helper<T>::get(default_val));
	user_def_types::get_init_value<T>(default_val));

?

[bader]

@keryell, I think @AlexeySachkov is seeking for discussion of this change in this comment: #856 (comment)

However, I do like the new name better, because with the old name one might think that function is returning some kind of helper instead of an init value. Let me know about the preference, I can rework the PR accordingly.

[keryell]

I do not like the word helper which does not help the understanding. :-)

[AlexeySachkov]

What is the value of having struct instead of generic functions like

Yes, that's a good point. We can still have it as a function, but we can also rename it along the way, I will update the PR

[AlexeySachkov]

Refactored back to function, removed helper from the name in 6211bfc

[keryell]

Thanks for simplification!