Some attributes are only valid on function items. When checking these
attributes, codegen_attrs previously sometimes called `fn_sig` on the
item they were attached to without first ensuring that the item was a
function. This led to an ICE (#105594), since `fn_sig` can
only be called on functions.
After this change, we skip calling `fn_sig` if the item the attribute is
attached to must be a function but invalidly isn't, because `check_attr`
will reject such cases without codegen_attrs's intervention.
