libstdc++: Fix chrono::__detail::ceil to work with C++11

In C++11 constexpr functions can only have a return statement, so we
need to fix __detail::ceil to make it valid in C++11. This can be done
by moving the comparison and increment into a new function, __ceil_impl,
and calling that with the result of the duration_cast.

This would mean the standard C++17 std::chrono::ceil function would make
two further calls, which would add too much overhead when not inlined.
For C++17 and later use a using-declaration to add chrono::ceil to
namespace __detail. For C++11 and C++14 define chrono::__detail::__ceil
as a C++11-compatible constexpr function template.

libstdc++-v3/ChangeLog:

	* include/std/chrono [C++17] (chrono::__detail::ceil): Add
	using declaration to make chrono::ceil available for internal
	use with a consistent name.
	(chrono::__detail::__ceil_impl): New function template.
	(chrono::__detail::ceil): Use __ceil_impl to compare and
	increment the value. Remove SFINAE constraint.

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