[libc++] Add _VSTD:: qualifications to ADL-proof <algorithm>.
Relevant blog post: https://quuxplusone.github.io/blog/2019/09/26/uglification-doesnt-stop-adl/
@ldionne: In the <algorithm> commit, I noticed that there are several places where otherwise-well-formed code has been commented out; e.g. look at the codepath in __nth_element following not_sorted:. The commented-out stuff dates back to @howard.hinnant's initial commit. Can I just remove the commented-out stuff, in a separate "NFC" commit?