'nonnull(1)' on a block parameter should apply to the block's argument.

Thanks to r199467, __attribute__((nonnull)) (without arguments) can apply
directly to parameters, instead of being applied to the whole function.
However, the old form of nonnull (with an argument index) could also apply
to the arguments of function and block pointers, and both of these can be
passed as parameters.

Now, if 'nonnull' with an argument is found on a parameter, /and/ the
parameter is a function or block pointer, it is handled the old way.

PR18795