Merge #6909

6909: Avoid querying attributes in item tree lowering r=jonas-schievink a=jonas-schievink

ItemTree is per-file, so there is no unique crate associated with it.
This means that it cannot know the active CfgOptions and thus couldn't
handle `cfg_attr`.

Prepare it for `cfg_attr`s by avoiding accessing attributes.

bors r+

Co-authored-by: Jonas Schievink <jonasschievink@gmail.com>