[libc++] Add _VSTD:: qualifications to ADL-proof <algorithm>.

Relevant blog post: https://quuxplusone.github.io/blog/2019/09/26/uglification-doesnt-stop-adl/

Differential Revision: https://reviews.llvm.org/D92776

GitOrigin-RevId: 1968804ac726e7674d5de22bc2204b45857da344
4 files changed