c++: local externs in templates do not get template head

[Nathan Sidwell]

Now we consistently mark local externs with DECL_LOCAL_DECL_P, we can
teach the template machinery not to give them a TEMPLATE_DECL head,
and the instantiation machinery treat them as the local specialiations
they are.  (openmp UDRs also fall into this category, and are dealt
with similarly.)

         gcc/cp/
         * pt.c (push_template_decl_real): Don't attach a template head to
         local externs.
         (tsubst_function_decl): Add support for headless local extern
         decls.
         (tsubst_decl): Add support for headless local extern decls.

pushed to trunk
-- 
Nathan Sidwell
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