[clang] [clang] add flow-sensitive nullability analysis for C/C++ (PR #189131)

[Chad Smith via cfe-commits]

cs01 wrote:

Please note this PR is in draft state. I plan to revisit the RFC and address the discussion points made there, but I believe this PR largely addresses them. I know your time is valuable, so please do not feel obligated to review this code in its current draft state as I plan to update it more, and address any failures. 

That said, The changes I will make to this PR will likely be incremental and not drastic, so if you want to take a look or test it out it will likely be applicable to the RFC discussion. Thank you in advance for your consideration. 

https://github.com/llvm/llvm-project/pull/189131