c++: maybe_substitute_reqs_for fix
While working on PR109751 I found that maybe_substitute_reqs_for was doing
the wrong thing for a non-template friend, substituting in the template args
of the scope's original template rather than those of the instantiation.
This didn't end up being necessary to fix the PR, but it's still an
improvement.
gcc/cp/ChangeLog:
* pt.cc (outer_template_args): Handle non-template argument.
* constraint.cc (maybe_substitute_reqs_for): Pass decl to it.
* cp-tree.h (outer_template_args): Adjust.