[c++1z] P0490R0, NB comment GB 20: if std::tuple_size<T> is complete, use the

tuple-like interpretation of decomposition declaration even if there is no
::value member. We already did this, anticipating this resolution, just update
comments and tweak a testcase.


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@289021 91177308-0d34-0410-b5e6-96231b3b80d8