This commit removes the `-D warnings` flag being passed through the makefiles to
all crates to instead be a crate attribute. We want these attributes always
applied for all our standard builds, and this is more amenable to Cargo-based
builds as well.
Note that all `deny(warnings)` attributes are gated with a `cfg(stage0)`
attribute currently to match the same semantics we have today
rather being stored inline. Refactor (and rename) the visitor so that
(by default) it only visits the interior content of an item not nested
items.
This is a [breaking-change] for anyone who uses the HIR visitor. Besides
changing `visit::` to `intravisit::`, you need to refactor your visitor
in one of two ways, depending on what it requires:
1. If you just want to visit all items (most common), you should call
`krate.visit_all_items(&mut visitor)`.
2. If you need to visit nested items in the middle of the parent items,
you should override `visit_nested_item` with something like:
`self.visit_item(self.tcx.map.expect_item(item.id))`, presuming you
have access to a tcx (or at least a HIR map).
