Allow attributes on macros

[alexcrichton]

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.

[sanxiyn]

For archaeologists, stripping before expansion was done in #7297.

[huonw]

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.

Note that this requires modifying syntax::ext::expand::expand_item_mac so that macro_rules!s aren't dropped from the AST during expansion.