This attribute is currently only permitted on functions, but sometimes
it can be useful to opt out a specific local variable. That way the
function can still get a stack protector if other locals require it, but
locals which are opted out won't have the overhead of a stack protector
if not needed.
This is paired with an LLVM change which has already landed, which uses
the new metadata to opt specific alloca instructions out of requiring a
stack protector.
