c++: tighten TMPL_ARGS_LEVEL macro

c++: tighten TMPL_ARGS_LEVEL macro

This patch makes TMPL_ARGS_LEVEL verify the level argument is valid when
the one-dimensional vector case.  Doing so uncovered a couple of latent
issues: in try_class_unification, we weren't correctly copying targs
when it's two-dimensional, and in unify_pack_expansion it seems an
inequality test needs to be reversed.  This patch fixes both issues, and
in passing makes the former function free the temporary copy of targs.

gcc/cp/ChangeLog:

	* cp-tree.h (TMPL_ARGS_LEVEL): Assert LEVEL is 1 when
	TMPL_ARGS_HAVE_MULTIPLE_LEVELS is false.
	* pt.cc (try_class_unification): Correctly copy multidimensional
	targs.  Free the copy of targs.
	(unify_pack_expansion): Fix level comparison.