c++: CTAD for class tmpl defined inside partial spec [PR104294]

c++: CTAD for class tmpl defined inside partial spec [PR104294]

Here during deduction guide generation for the nested class template
B<char(int)>::C, the computation of outer_args yields the template
arguments relative to the primary template for B (i.e. {char(int)})
but what we really want is those relative to C's enclosing scope, the
partial specialization of B (i.e. {char, int}).

	PR c++/104294

gcc/cp/ChangeLog:

	* pt.cc (ctor_deduction_guides_for): Correct computation of
	outer_args.

gcc/testsuite/ChangeLog:

	* g++.dg/cpp1z/class-deduction106.C: New test.