[clang] Add more tests for _Countof (PR #133333)

[Alejandro Colomar via cfe-commits]

alejandro-colomar wrote:

@AaronBallman This is ready.  Still, my patches for GCC had another set of tests that are completely missing here: run-time tests (assert(3)) for non-ICE (so, VLA).  Maybe you could add a set of tests that are run (not just compiled).

https://github.com/llvm/llvm-project/pull/133333