Parse support for tuple patterns in var and let (#3448)

Co-authored-by: Richard Smith <richard@metafoo.co.uk>

toolchain/parse/handle_pattern.cpp

@@ -0,0 +1,18 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "toolchain/parse/context.h"
+
+namespace Carbon::Parse {
+
+auto HandlePattern(Context& context) -> void {
+  context.PopAndDiscardState();
+  if (context.PositionKind() == Lex::TokenKind::OpenParen) {
+    context.PushState(State::ParamListAsRegular);
+  } else {
+    context.PushState(State::BindingPattern);
+  }
+}
+
+}  // namespace Carbon::Parse