[C++] template parm cleanup

[Nathan Sidwell]

When working on 94827 I noticed that process_template_parm was doing 
more work than necessary -- using 'last_node' and later 'chainon'.  This 
fixes that up to just walk the list once.

Also, I didn't understand the reason why end_template_parms does a pop 
of the dummy header, and then a push of the real one -- why not just 
modify the dummy in place?  Added a comment about that.

pushed to trunk.

nathan
-- 
Nathan Sidwell
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