I noticed that the function context isn't set, and the
current_function_decl isn't updated by the time the attribute callbacks
execute. Thus I use current_function_decl, not
decl_function_context(node) to find out if the function is nested or
not. If this is not correct(I only checked the C frontend so far),
please suggest a better solution. The only other way I found was to
check the nestedness during code generation, which doesn't seem right to
me(and puts the error at the end of the function, not its declaration).