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_variable.cpp

@@ -13,7 +13,7 @@ auto HandleVariableIntroducer(Context& context, Parse::NodeId parse_node)
     -> bool {
   // No action, just a bracketing node.
   context.node_stack().Push(parse_node);
-  context.decl_state_stack().Push(DeclState::Var, parse_node);
+  context.decl_state_stack().Push(DeclState::Var);
   return true;
 }