[clang] [Clang][C2y] Add support for if declarations (N3356 paper) (PR #198244)

[Aaron Ballman via cfe-commits]

https://github.com/AaronBallman approved this pull request.

I expect there will be some follow-up work and we may discover some surprises once users throw real world code at us, but I think this is now ready to go. LGTM with a couple of minor nits addressed.

https://github.com/llvm/llvm-project/pull/198244