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

[Balanescu Eduard via cfe-commits]

avunit1 wrote:

Thanks for the review @NagyDonat! Addressed both points in the latest push (b28d566):

Moved the mock <coroutine> declarations out of the test into clang/test/Analysis/Inputs/system-header-simulator-cxx-coroutines.h, following the existing system-header-simulator-cxx-*.h convention.
Replaced __UINTPTR_TYPE__/uintptr_t with a plain unsigned in the reproducer — the exact integer type was never load-bearing for the bug, so this avoids the portability question entirely.

Also rebased onto current main to clear the "unable to merge" state, so the branch is now a single commit again.
Still waiting on the CFG.cpp review — happy to answer questions on the CFG construction approach in the meantime if that helps.

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