PR c++/61825
	* c-family/c-common.c (handle_alias_ifunc_attribute): Check
	that visibility change is possible
	(handle_weakref_attribute): Likewise.
	* cgraph.h (symtab_node): Add method get_create and
	field refuse_visibility_changes.
	(symtab_node::get_create): New method.
	* fold-const.c (tree_single_nonzero_warnv_p): Use get_create.
	* varasm.c (mark_weak): Verify that visibility change is
	possible.

	* gcc.dg/tree-ssa/nonzero-1.c: Require error to be output.

From-SVN: r215409