[clang] Model coroutine promise construction in the CFG (PR #208569)

[Donát Nagy via cfe-commits]

https://github.com/NagyDonat commented:

The code looks good to me at first glance, but I'm not familiar with the CFG construction, so please wait for another review of the change in `CFG.cpp`.

(If there is no other reviewer within 1 week, ping me, and I'll try to learn the logic of the CFG construction process to be able to review that part of the change. 

https://github.com/llvm/llvm-project/pull/208569