Move add invalid node. Follow up to carbon-language#3473. (carbon-lan…

…guage#3491)

I think this matches expectations about how the parse code should be
structured better.

toolchain/parse/handle_impl.cpp

@@ -34,6 +34,11 @@ auto HandleImplAfterIntroducer(Carbon::Parse::Context& context) -> void {
  "Expected `[` after `forall` in `impl` declaration.");
  context.emitter().Emit(*context.position(), ImplExpectedAfterForall);
  context.ReturnErrorOnState();
+ // If we aren't producing a node from the PatternListAsImplicit state,
+ // we still need to create a node to be the child of the `ImplForall`
+ // token created in the `ImplAfterForall` state.
+ context.AddLeafNode(NodeKind::InvalidParse, *context.position(),
+ /*has_error=*/true);
  }
  } else {
  // One of:
@@ -46,8 +51,6 @@ auto HandleImplAfterIntroducer(Carbon::Parse::Context& context) -> void {
 auto HandleImplAfterForall(Carbon::Parse::Context& context) -> void {
  auto state = context.PopState();
  if (state.has_error) {
- context.AddLeafNode(NodeKind::InvalidParse, *context.position(),
- /*has_error=*/true);
  context.ReturnErrorOnState();
  }
  context.AddNode(NodeKind::ImplForall, state.token, state.subtree_start,