c++: fix TTP level reduction cache

c++: fix TTP level reduction cache

We try to cache the result of reduce_template_parm_level so that when we
reduce the same parm multiple times we get the same result, but this wasn't
working for template template parms because in that case TYPE is a
TEMPLATE_TEMPLATE_PARM, and so same_type_p was false because of the same
level mismatch that we're trying to adjust for.  So in that case compare the
template parms of the template template parms instead.

The result can be seen in nontype12.C, where we previously gave three
duplicate errors on line 7 and now give only one because subsequent
substitutions use the cache.

gcc/cp/ChangeLog:

	* pt.cc (reduce_template_parm_level): Fix comparison of
	template template parm to cached version.

gcc/testsuite/ChangeLog:

	* g++.dg/template/nontype12.C: Check for duplicate error.