[PATCH] Implement std::experimental::sample

[Marshall Clow]

Two more comments:

1. In the libc++ test directory (support), there's a file full of iterator adapters that take a `int *` (say), and turn it into a forward iterator (for example). Makes writing tests simpler.

2. Here's another test candidate: `PopulationIteartor`'s value type shall be writeable to `out`.

That means that you should be able to generate a sample of (say) doubles from a population of ints.


http://reviews.llvm.org/D9044

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