c++: passing one ttp to another [PR108179]

I kept trying to improve our choice of how many levels of outer_args to add,
when really the problem was that outer_args are for PARM and for this
reverse deduction we should be adding the outer arguments for ARG.

I spent quite a while trying to get DECL_CONTEXT set consistently on
template template parameters that have gone through
reduce_template_parm_level before I realized I could just use
current_scope().

	PR c++/108179
	PR c++/104107
	PR c++/95036

gcc/cp/ChangeLog:

	* pt.cc (coerce_template_template_parms): Use args from
	DECL_CONTEXT (arg_tmpl) instead of outer_args.

gcc/testsuite/ChangeLog:

	* g++.dg/template/ttp35.C: New test.