Use sym::dummy in one more place.

It makes it clearer that the symbol is unused and doesn't matter.

Author: nnethercote

compiler/rustc_resolve/src/diagnostics.rs

@@ -1091,7 +1091,7 @@ impl<'ra, 'tcx> Resolver<'ra, 'tcx> {
                     ));
                 }
                 Scope::BuiltinAttrs => {
-                    let res = Res::NonMacroAttr(NonMacroAttrKind::Builtin(kw::Empty));
+                    let res = Res::NonMacroAttr(NonMacroAttrKind::Builtin(sym::dummy));
                     if filter_fn(res) {
                         suggestions.extend(
                             BUILTIN_ATTRIBUTES

compiler/rustc_span/src/hygiene.rs

@@ -1439,7 +1439,8 @@ pub fn decode_syntax_context<D: Decoder, F: FnOnce(&mut D, u32) -> SyntaxContext
                 }
             }
             Entry::Vacant(entry) => {
-                // We are the first thread to start decoding. Mark the current thread as being progress.
+                // We are the first thread to start decoding. Mark the current thread as being
+                // progress.
                 context.local_in_progress.borrow_mut().insert(raw_id);
 
                 // Allocate and store SyntaxContext id *before* calling the decoder function,