Implement LWG Issue: 2280. begin/end for arrays should be constexpr and noexcept, plus a drive-by fix for cbegin/cend suggested by Peter Sommerlad.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@201703 91177308-0d34-0410-b5e6-96231b3b80d8