Avoid referring to the item tree except in the def map

Item tree IDs are very unstable (adding an item of a kind invalidates all following items of the same kind). Instead use ast ids, which, since the previous commit, are pretty stable.

crates/hir-def/src/item_tree/lower.rs

@@ -626,7 +626,7 @@ fn private_vis() -> RawVisibility {
     )
 }
 
-fn visibility_from_ast(
+pub(crate) fn visibility_from_ast(
     db: &dyn DefDatabase,
     node: Option<ast::Visibility>,
     span_for_range: &mut dyn FnMut(::tt::TextRange) -> SyntaxContext,