[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