Rollup merge of #124214 - carbotaniuman:parse_unsafe_attrs, r=michaelwoerister

Parse unsafe attributes

Initial parse implementation for #123757

This is the initial work to parse unsafe attributes, which is represented as an extra `unsafety` field in `MetaItem` and `AttrItem`. There's two areas in the code where it appears that parsing is done manually and not using the parser stuff, and I'm not sure how I'm supposed to thread the change there.

compiler/rustc_ast_lowering/src/lib.rs

@@ -905,6 +905,7 @@ impl<'a, 'hir> LoweringContext<'a, 'hir> {
         let kind = match attr.kind {
             AttrKind::Normal(ref normal) => AttrKind::Normal(P(NormalAttr {
                 item: AttrItem {
+                    unsafety: normal.item.unsafety,
                     path: normal.item.path.clone(),
                     args: self.lower_attr_args(&normal.item.args),
                     tokens: None,