They don't work with std::span, since std::span has neither
const_iterator nor cbegin()/cend().
To fix, perfectly forward the return type with decltype(auto) instead
of mentioning T::const_iterator, and rely on the const-qualfication of
the help::c{r,}{begin,end}() functions' arguments to select the
correct T::{r,}{begin,end}() overload.
Change-Id: I4992d4bd521d2dc0f9ea51ae70cde8286ae543a5
Reviewed-by: Mårten Nordheim <marten.nordheim@qt.io>
dd4e96df52