c++: Fix static local vars in extern "C".

c++: Fix static local vars in extern "C".

Since my patch for PR 91476 moved visibility determination sooner, a local
static in a vague linkage function now gets TREE_PUBLIC set before
retrofit_lang_decl calls set_decl_linkage, which was making decl_linkage
think that it has external linkage.  It still has no linkage according to
the standard.

gcc/cp/ChangeLog
2020-02-13  Jason Merrill  <jason@redhat.com>

	PR c++/93643
	PR c++/91476
	* tree.c (decl_linkage): Always lk_none for locals.