[libcxx] [test] Fix -Wmismatched-tags in tuple_size_structured_bindings.pass.cpp.

Clang and C1XX both complain about mismatched class/struct, but libc++ and MSVC's STL
differ on what they use for tuple_element/tuple_size, so there's no way to win here.

I'm reverting this part of my previous change. In the future, I'll have to suppress
the warning for one compiler or the other.

llvm-svn: 305854