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.cpp

@@ -8,11 +8,12 @@
 namespace Carbon::Parse {
 
 auto HandlePattern(Context& context) -> void {
-  context.PopAndDiscardState();
+  auto state = context.PopState();
   if (context.PositionKind() == Lex::TokenKind::OpenParen) {
-    context.PushState(State::PatternListAsTuple);
+    context.PushStateForPattern(State::PatternListAsTuple,
+                                state.in_var_pattern);
   } else {
-    context.PushState(State::BindingPattern);
+    context.PushStateForPattern(State::BindingPattern, state.in_var_pattern);
   }
 }