[clang] [C2y] Implement WG14 N3369 and N3469 (_Countof) (PR #133125)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> Thanks @AaronBallman !! I am still working on my patch, so just for learning about Clang internals, I'll try to finish mine, and compare it to yours. :-)

Sorry for stealing this one out from under you! It turns out it was a bit more involved than I expected because I didn't realize how we represented multidimensional VLAs in the type system.

> Sorry for being so slow!

No worries! I wasn't certain if you had gotten busy or not, and I had some spare bandwidth and wanted to write code for a bit.

https://github.com/llvm/llvm-project/pull/133125