Fix several tuple bugs that were exposed by clang's implementation of CWG 1402.  This fixes http://llvm.org/bugs/show_bug.cgi?id=17798.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@194154 91177308-0d34-0410-b5e6-96231b3b80d8