c++: access checking in aggregate initialization [PR96673]

c++: access checking in aggregate initialization [PR96673]

We were deferring access checks while parsing B<int>{}, didn't adjust that
when we went to instantiate the default member initializer for B::c,
deferred access checking for C::C, and then checked it after parsing
B<int>{}, back in the main() context which has no access.  We need to do the
access checks in the class context of the DMI.

I tried fixing this in push_to/pop_from_top_level, but that caused several
regressions.

gcc/cp/ChangeLog:

	PR c++/96673
	* init.c (get_nsdmi): Don't defer access checking.

gcc/testsuite/ChangeLog:

	PR c++/96673
	* g++.dg/cpp1y/nsdmi-aggr13.C: New test.