[libc++][C++17] Elementary string conversions for integral types
Major QoI considerations:
- The facility is backported to C++14, same as libstdc++.
- Efforts have been made to minimize the header dependencies.
- The design is friendly to the uses of MSVC intrinsics (__emulu, _umul128, _BitScanForward, _BitScanForward64) but not implemented; future contributions are welcome.
Thanks to Milo Yip for contributing the implementation of __u64toa and __u32toa.
Reviewers: mclow.lists, EricWF
Reviewed By: mclow.lists
Subscribers: ldionne, Quuxplusone, christof, mgorny, cfe-commits
Differential Revision: https://reviews.llvm.org/D41458