Implement std::launder for C++17
* doc/xml/manual/status_cxx2017.xml: Update status.
* doc/html/*: Regenerate.
* include/std/type_traits (has_unique_object_representations): Guard
with __has_builtin check.
* libsupc++/new (launder): Define for C++17.
* testsuite/18_support/launder/1.cc: New test.
* testsuite/18_support/launder/requirements.cc: New test.
* testsuite/18_support/launder/requirements_neg.cc: New test.
git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@241648 138bc75d-0d04-0410-961f-82ee72b054a4