c++: ICE with concepts TS multiple auto deduction [PR101886]

c++: ICE with concepts TS multiple auto deduction [PR101886]

In extract_autos_r, we need to recompute TYPE_CANONICAL for the template
type parameter after adjusting its index, otherwise we end up with a
comptypes ICE for the below testcase.  Note that such in-place type
adjustment isn't generally safe to do since the type could be the
TYPE_CANONICAL of another (unadjusted) type, but in this case the
canonical auto (of some level and 0 index) is the first auto (of that
level) that's created, and so any auto that we do end up adjusting can't
be the canonical one.

	PR c++/101886

gcc/cp/ChangeLog:

	* pt.cc (extract_autos_r): Recompute TYPE_CANONICAL after
	adjusting the template type parameter's index.  Simplify
	by using TEMPLATE_TYPE_IDX.  Add some sanity checks.

gcc/testsuite/ChangeLog:

	* g++.dg/concepts/auto5.C: New test.