libstdc++: Allow std::swap to find overload for std::exception_ptr

libstdc++: Allow std::swap to find overload for std::exception_ptr

The non-member swap for std::exception_ptr is in a nested namespace and
so can only be found by ADL currently. Add a using-declaration so that
qualified std::swap calls will use the std::exception_ptr::swap member,
instead of the generic std::swap.

There's no new test for this, because the generic std::swap works, it
just does more work than is necessary.

Also tell Doxygen to replace the __exception_ptr namespace with
"__unspecified__" in the generate docs, so the real name is not
documented.

libstdc++-v3/ChangeLog:

	* doc/doxygen/user.cfg.in (PREDEFINED): Replace __exception_ptr
	with "__unspecified__".
	* libsupc++/exception_ptr.h: Improve doxygen docs.
	(__exception_ptr::swap): Also declare in namespace std.