Use correct spans when rewrting attributes

[topecongiro]

This PR

1. add changes to prevent rustfmf to overflow arithmetic when running tests.
2. try to use correct spans when rewriting attributes. The span of ast::Expr does not cover its attributes, and the span of ast::MetaItem is missing closing parens.

[nrc]

I would like to makes sure we fix this in libsyntax and then remove this fix from the rustfmt code - i.e., ensure that it stays temporary. Probably the best way to do that is file an issue here and add the issue number to the FIXME.