[Clang][NFC] Add tests for llvm#56071

Theses tests were part of https://reviews.llvm.org/D140184,
which is no longer necessary but preserving the tests seems useful.

Thanks to Richard Trieu for providing these tests and the work
on this PR.

Author: cor3ntin

clang/test/SemaCXX/lambda-expressions-gh56071.cpp

@@ -0,0 +1,51 @@
+// RUN: %clang_cc1 -fsyntax-only -verify %s -Wno-unused-value -std=c++17
+// expected-no-diagnostics
+namespace test1 {
+
+template <int num> int return_num() { return num; }
+
+template <typename lambda> struct lambda_wrapper {
+  lambda &outer_lambda;
+  lambda_wrapper(lambda& outer_lambda) : outer_lambda(outer_lambda) {}
+  template <typename T> auto operator+(T t) { outer_lambda(t); return 1; }
+};
+
+template <int... nums, typename lambda>
+void bw(lambda& outer_lambda) {
+  (lambda_wrapper(outer_lambda) + ... + return_num<nums>());
+}
+
+template <typename lambda> auto check_return_type(lambda inner_lambda) {
+  using inner_lambda_return_type = decltype(inner_lambda(5));
+}
+
+void cs() {
+  auto outer_lambda = [](auto param) {
+    auto inner_lambda = [](auto o) -> decltype(param) {};
+    check_return_type(inner_lambda);
+  };
+  bw<1,2>(outer_lambda);
+}
+
+}  // namespace test1
+
+namespace test2 {
+
+template <typename lambda>
+auto run_lambda_with_zero(lambda l) {
+  l(0);
+}
+template <typename ... Ts, typename lambda>
+void run_lambda_once_per_type(lambda l) {
+  ((Ts{}, run_lambda_with_zero(l)), ...);
+}
+template <typename> void inner_function() {
+  char c;
+  [](auto param) -> decltype(c) { return param; }(0);
+}
+void run() {
+  auto x = [](auto) -> void { inner_function<int>(); };
+  run_lambda_once_per_type<int>(x);
+}
+
+}  // namespace test2