[NFC][Coroutines] Add regression test for heap allocation elision optimization

Recently C++ snippet included in this patch popped up at least twice in different regression contexts:
https://github.com/llvm/llvm-project/issues/56262 and https://reviews.llvm.org/D123300
It appears that Clang users rely on HALO so adding C++ example coming originally from Gor Nishanov to tests
should help in avoiding similar regressions in future.

Differential Revision: https://reviews.llvm.org/D129279

clang/test/CodeGenCoroutines/Inputs/numeric.h

@@ -0,0 +1,10 @@
+#pragma once
+
+namespace std {
+template <class InputIterator, class T>
+T accumulate(InputIterator first, InputIterator last, T init) {
+  for (; first != last; ++first)
+    init = init + *first;
+  return init;
+}
+} // namespace std

clang/test/CodeGenCoroutines/coro-halo.cpp

@@ -0,0 +1,102 @@
+// This tests that the coroutine heap allocation elision optimization could happen succesfully.
+// RUN: %clang_cc1 -no-opaque-pointers -triple x86_64-unknown-linux-gnu -std=c++20 -O2 -emit-llvm %s -o - | FileCheck %s
+
+#include "Inputs/coroutine.h"
+#include "Inputs/numeric.h"
+
+template <typename T> struct generator {
+  struct promise_type {
+    T current_value;
+    std::suspend_always yield_value(T value) {
+      this->current_value = value;
+      return {};
+    }
+    std::suspend_always initial_suspend() { return {}; }
+    std::suspend_always final_suspend() noexcept { return {}; }
+    generator get_return_object() { return generator{this}; };
+    void unhandled_exception() {}
+    void return_void() {}
+  };
+
+  struct iterator {
+    std::coroutine_handle<promise_type> _Coro;
+    bool _Done;
+
+    iterator(std::coroutine_handle<promise_type> Coro, bool Done)
+        : _Coro(Coro), _Done(Done) {}
+
+    iterator &operator++() {
+      _Coro.resume();
+      _Done = _Coro.done();
+      return *this;
+    }
+
+    bool operator==(iterator const &_Right) const {
+      return _Done == _Right._Done;
+    }
+
+    bool operator!=(iterator const &_Right) const { return !(*this == _Right); }
+    T const &operator*() const { return _Coro.promise().current_value; }
+    T const *operator->() const { return &(operator*()); }
+  };
+
+  iterator begin() {
+    p.resume();
+    return {p, p.done()};
+  }
+
+  iterator end() { return {p, true}; }
+
+  generator(generator const &) = delete;
+  generator(generator &&rhs) : p(rhs.p) { rhs.p = nullptr; }
+
+  ~generator() {
+    if (p)
+      p.destroy();
+  }
+
+private:
+  explicit generator(promise_type *p)
+      : p(std::coroutine_handle<promise_type>::from_promise(*p)) {}
+
+  std::coroutine_handle<promise_type> p;
+};
+
+template <typename T>
+generator<T> seq() {
+  for (T i = {};; ++i)
+    co_yield i;
+}
+
+template <typename T>
+generator<T> take_until(generator<T> &g, T limit) {
+  for (auto &&v : g)
+    if (v < limit)
+      co_yield v;
+    else
+      break;
+}
+
+template <typename T>
+generator<T> multiply(generator<T> &g, T factor) {
+  for (auto &&v : g)
+    co_yield v *factor;
+}
+
+template <typename T>
+generator<T> add(generator<T> &g, T adder) {
+  for (auto &&v : g)
+    co_yield v + adder;
+}
+
+int main() {
+  auto s = seq<int>();
+  auto t = take_until(s, 10);
+  auto m = multiply(t, 2);
+  auto a = add(m, 110);
+  return std::accumulate(a.begin(), a.end(), 0);
+}
+
+// CHECK-LABEL: define{{.*}} i32 @main(
+//   CHECK: ret i32 1190
+//   CHECK-NOT: call{{.*}}_Znwm