shared_mutex (__shared_mutex_pthread, [...]): New helper types implementing the shared mutex requirements.
* include/std/shared_mutex (__shared_mutex_pthread,
__shared_mutex_cv): New helper types implementing the shared mutex
requirements.
(shared_mutex): New type for C++17.
(shared_timed_mutex): Derive from one of the new helper types.
* testsuite/30_threads/shared_mutex/cons/1.cc: New.
* testsuite/30_threads/shared_mutex/cons/assign_neg.cc: New.
* testsuite/30_threads/shared_mutex/cons/copy_neg.cc: New.
* testsuite/30_threads/shared_mutex/requirements/standard_layout.cc:
New.
* testsuite/30_threads/shared_mutex/try_lock/1.cc: New.
* testsuite/30_threads/shared_mutex/try_lock/2.cc: New.
From-SVN: r224158
