[PATCH] Implement std::experimental::sample

[Eric Fiselier]

Since it is possible that we will not guard `std::experimental::random_sample` in a C++14 only block the tests should compile in C++03. Here is my patch to get them working. https://gist.github.com/EricWF/bef9f2a3ca4c86cb0781.

PS. you can pass `--param=std=c++03` when invoking LIT to run the tests in C++03. Example:

  lit -sv --param=std=c++03 test/std/experimental/algorithm


http://reviews.llvm.org/D9044

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