libstdc++: Use _M_get() in std::optional internals

libstdc++: Use _M_get() in std::optional internals

Now that _base::_M_get() doesn't check the precondition, we can use
_M_get() instead of operator*() for the internal uses where we've
already checked the precondition holds.

Add a using-declaration so that we don't need to lookup _M_get in the
dependent base class, and make optional<U> a friend so that the
converting constructors and assignment operators can use the parameter's
_M_get member.

libstdc++-v3/ChangeLog:

	* include/std/optional (optional): Add using-declaraction for
	_Base::_M_get and declare optional<U> as friend.
	(optional(const optional<U>&)): Use
	_M_get instead of operator*.
	(optional(optional<U>&&)): Likewise.
	(operator=(const optional<U>&)): Likewise.
	(operator=(optional<U>&&)): Likewise.
	(and_then, tansform): Likewise.