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

[Balázs Benics via cfe-commits]

https://github.com/steakhal commented:

One more note. Anything affecting the CFG, we should have a DumpCFG test using FileCheck ensuring that the relative order of the operations match our expectations. Things can get very subtle in the CFG. Many moving hidden parts. We should watch out for those and pin what matters.

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