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.