Allow attributes on macros

It's unclear to me why these currently aren't allowed, and my best guess is that
a long time ago we didn't strip the ast of cfg nodes before syntax expansion.
Now that this is done, I'm not certain that we should continue to prohibit this
functionality.

This is a step in the right direction towards #5605, because now we can add an
empty `std::macros` module to the documentation with a bunch of empty macros
explaining how they're supposed to be used.