Mark LWG 2716 as complete - shuffle and sample disallows lvalue URNGs.

Mark LWG  2716 as complete - shuffle and sample disallows lvalue URNGs.

Libc++'s implementation of shuffle and sample already support lvalue and rvalue
RNG's. This patch adds tests for both categories and marks the issue as complete.

This patch also contains drive-by change for std::experimental::sample which
improves the diagnostics produced when the correct iterator categories are
not supplied.

llvm-svn: 279947