Mark LWG #2314. 'apply() should return decltype(auto) and use decay_t before tuple_size' as complete. This is a correction to some example code in the standard, no change needed for libc++.

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@203579 91177308-0d34-0410-b5e6-96231b3b80d8