Underline the complete declaration in diagnostics (#3508)

Builds upon @domisterwoozy 's excellent #3442 . Removes the need to
store the first node of a declaration in the declaration state stack.

toolchain/check/handle_modifier.cpp

@@ -57,9 +57,6 @@ static auto HandleModifier(Context& context, Parse::NodeId parse_node,
   } else {
     s.modifier_set |= keyword;
     saw_modifier = parse_node;
-    if (is_access || !s.saw_access_modifier.is_valid()) {
-      s.first_node = parse_node;
-    }
   }
   return true;
 }