libstdc++: Add nodiscard attribute to mutex try_lock functions

libstdc++: Add nodiscard attribute to mutex try_lock functions

libstdc++-v3/ChangeLog:

	* include/bits/std_mutex.h (mutex): Add nodiscard attribute to
	try_lock member function.
	* include/bits/unique_lock.h (unique_lock): Likewise for
	try_lock, try_lock_until, try_lock_for member functions, and
	owns_lock and mutex member functions.
	* include/std/mutex (recursive_mutex): Likewise for try_lock
	member function.
	(timed_mutex, recursive_timed_mutex, try_lock): Likewise for
	try_lock, try_lock_until, try_lock_for member functions.
	(try_lock): Likewise for non-member function.
	* include/std/shared_mutex (shared_mutex): Likewise for try_lock
	and try_lock_shared member functions.
	(shared_timed_mutex): Likewise for try_lock, try_lock_for,
	try_lock_shared, try_lock_shared_for, try_lock_until, and
	try_lock_shared_until member functions.
	(shared_lock): Likewise for try_lock, try_lock, try_lock_for,
	try_lock_until, owns_lock, and mutex member functions.
	* testsuite/30_threads/recursive_timed_mutex/try_lock_until/clock_neg.cc:
	Cast discarded value expression to void.
	* testsuite/30_threads/shared_lock/locking/3.cc: Likewise.
	* testsuite/30_threads/shared_lock/locking/4.cc: Likewise.
	* testsuite/30_threads/shared_lock/locking/clock_neg.cc:
	Likewise.
	* testsuite/30_threads/shared_timed_mutex/try_lock_until/clock_neg.cc:
	Likewise.
	* testsuite/30_threads/timed_mutex/try_lock_until/clock_neg.cc:
	Likewise.
	* testsuite/30_threads/try_lock/4.cc: Likewise.
	* testsuite/30_threads/unique_lock/cons/60497.cc: Likewise.
	* testsuite/30_threads/unique_lock/locking/3.cc: Likewise.
	* testsuite/30_threads/unique_lock/locking/clock_neg.cc:
	Likewise.