Separate node kind for bindings inside var (#4822)

This is a step toward making binding pattern handling more robust, by
removing its reliance on the node stack for context.

toolchain/parse/handle_pattern_list.cpp

@@ -10,10 +10,10 @@ namespace Carbon::Parse {
 // Handles PatternListElementAs(Implicit|Tuple).
 static auto HandlePatternListElement(Context& context, State pattern_state,
                                      State finish_state) -> void {
-  context.PopAndDiscardState();
+  auto state = context.PopState();
 
-  context.PushState(finish_state);
-  context.PushState(pattern_state);
+  context.PushStateForPattern(finish_state, state.in_var_pattern);
+  context.PushStateForPattern(pattern_state, state.in_var_pattern);
 }
 
 auto HandlePatternListElementAsImplicit(Context& context) -> void {
@@ -39,7 +39,7 @@ static auto HandlePatternListElementFinish(Context& context,
   if (context.ConsumeListToken(NodeKind::PatternListComma, close_token,
                                state.has_error) ==
       Context::ListTokenKind::Comma) {
-    context.PushState(param_state);
+    context.PushStateForPattern(param_state, state.in_var_pattern);
   }
 }
 
@@ -58,13 +58,13 @@ static auto HandlePatternList(Context& context, NodeKind node_kind,
                               Lex::TokenKind open_token_kind,
                               Lex::TokenKind close_token_kind,
                               State param_state, State finish_state) -> void {
-  context.PopAndDiscardState();
+  auto state = context.PopState();
 
-  context.PushState(finish_state);
+  context.PushStateForPattern(finish_state, state.in_var_pattern);
   context.AddLeafNode(node_kind, context.ConsumeChecked(open_token_kind));
 
   if (!context.PositionIs(close_token_kind)) {
-    context.PushState(param_state);
+    context.PushStateForPattern(param_state, state.in_var_pattern);
   }
 }