[C++/58583] ICE instantiating NSDMIs

[Nathan Sidwell]

This patch fixes 58582, a set of ICEs that happen instantiating NSDMIs.  There 
are a couple of causes, both fixed.

1) instantiating the  template while parsing an NSDMI of the template itself. 
We see a DEFAULT_ARG in get_nsdmi.  Fixed  by jumping to the existing error 
handling for the  non-template case.

2) recursive instantiation of the NSDMI itself.  As we instantiate lazily, we 
end up running out of stack.  Fixed by creating a temporary DEFAULT_ARG and then 
detecting it on a subsequent recursion.  I did contemplate having this direct to 
the same error as above, but decided against it, because this really is an 
instantiation problem not a parsing problem.

built & tested on x86_64-linux, ok?

nathan
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