c++: alias template template_info setting

c++: alias template template_info setting

During the construction of alias templates we can alter its
template_info.  This is really weird, because that's morally immutable
data.  In this case it's ok, but let's not create a duplicate
template_info, and add asserts to make sure it is changing in exactly
the way we expect.

	gcc/cp/
	* cp-tree.h (SET_TYPE_TEMPLTE_INFO): Do not deal with ALIAS templates.
	* pt.c (lookup_template_class_1): Special-case alias template
	template_info setting.