re PR c++/61825 (g++.dg/cpp0x/static_assert9.C FAILs)
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.