[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).
> 
> We don't do end-to-end testing in Clang directly (those kinds of tests are sometimes more fragile because of differences between host platforms); we usually shunt that sort of thing off into llvm-test-suite, which gets less visibility and has less post-commit bot coverage, so we typically only do that for special cases. Instead we test what LLVM IR we emit and we rely on the backend tests to verify that the given LLVM IR instructions behave as intended.

Thanks!  Makes sense.

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