[libstdc++, C++17] Implement C++17 P0330 size_t UDL.

[Jonathan Wakely]

On 21/07/16 00:18 -0400, Ed Smith-Rowland wrote:
>This patch defines
>
>  operator""zu(unsigned long long __n)
>
>for size_t literals.
>
>for (auto k = 0zul; k < v.size(); ++k)
>
>   ...
>
>
>Testing on x86-64-linux is finishing but I'm past these tests.
>
>OK?

P0330 isn't in C++17. In Oulu LEWG voted to forward it to LWG for
C++Next (i.e. C++20) and LFTSv3. I don't think LWG reviewed it yet.

https://issues.isocpp.org/show_bug.cgi?id=77