c++: partial ordering with dependent NTTP type [PR105289]

c++: partial ordering with dependent NTTP type [PR105289]

Here ever since r11-6483-ge2e2f3f2c9400f we're rejecting and crashing
on (respectively) two testcases that we used to accept in C++17 mode
since r8-1437-g3da557ec145823.  Both testcases declare a partial
specialization of a primary template that has an NTTP with dependent
type, and the validity of these partial specializations is unclear and
the subject of PR86193 / CWG 455.

So for now, this minimal patch just aims to fix the crash in the second
testcase.  During deduction, when checking whether the type of an NTTP
uses still-undeduced parameters, we were incorrectly substituting into
the previously substituted type instead of into its original type.

In passing this patch also downgrades the "not more specialized"
diagnostic from a permerror to a pedwarn.

	PR c++/105289
	PR c++/86193

gcc/cp/ChangeLog:

	* pt.cc (process_partial_specialization): Downgrade "partial
	specialization isn't more specialized" diagnostic from permerror
	to an on-by-default pedwarn.
	(unify) <case TEMPLATE_PARM_INDEX>: When substituting into the
	NTTP type a second time, use the original type not the
	substituted type.

gcc/testsuite/ChangeLog:

	* g++.dg/template/partial-specialization11.C: New test.
	* g++.dg/template/partial-specialization12.C: New test.