[clang] [llvm] [C2y] Support WG14 N3457, the __COUNTER__ macro (PR #162662)

[David Spickett via llvm-commits]

DavidSpickett wrote:

I think we need to disable/downgrade a warning over in llvm-test-suite:
```
/home/tcwg-buildbot/worker/clang-aarch64-sve2-vla/test/test-suite/MicroBenchmarks/libs/benchmark/include/benchmark/benchmark.h:1534:49: error: '__COUNTER__' is a C2y extension [-Werror,-Wc2y-extensions]
```
https://lab.llvm.org/buildbot/#/builders/198/builds/9380

(I don't have time to do to it myself right now)

https://github.com/llvm/llvm-project/pull/162662