Implement void_t from N3911. Add a private version for use in the library before C++1z. Update the 1z status page, marking a bunch of issues that don't require library changes as complete (2129, 2212, 2230, 2233, 2325, 2365, 2376)

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@222138 91177308-0d34-0410-b5e6-96231b3b80d8