c++: Remove a broken error-recovery path

[Nathan Sidwell]

The remaining use of xref_tag_from_type was also suspicious.  It turns
out to be an error path.  At parse time we diagnose that a class
definition cannot appear, but we swallow the definition.  This code
was attempting to push it into the global scope (or find a conflict).
This seems needless, just return error_mark_node.  This was the
simpler fix than going through the parser and figuring out how to get
it to put in error_mark_node at the right point.

         gcc/cp/
         * cp-tree.h (xref_tag_from_type): Don't declare.
         * decl.c (xref_tag_from_type): Delete.
         * pt.c (lookup_template_class_1): Erroneously located class
         definitions just give error_mark, don't try and inject it into the
         namespace.

pushed to trunk

-- 
Nathan Sidwell
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