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

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

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.