Auto merge of #122371 - oli-obk:visit_nested_body, r=tmiasko

Stop walking the bodies of statics for reachability, and evaluate them instead

cc `@saethlin` `@RalfJung`

cc #119214

This reuses the `DefIdVisitor` from `rustc_privacy`, because they basically try to do the same thing.

This PR's changes can probably be extended to constants, too, but let's tackle that separately, it's likely more involved.

Author: bors