Implement n3658 - Compile-time integer sequences

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185343 91177308-0d34-0410-b5e6-96231b3b80d8
Implement n3656 - make_unique. Thanks to Howard for the review and suggestions.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@185352 91177308-0d34-0410-b5e6-96231b3b80d8