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

[Aaron Ballman via cfe-commits]

AaronBallman 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)

That's the intent behind this change: https://github.com/llvm/llvm-project/pull/162662/files#diff-8f3dca342ade6d0034ad6afe0909f24fbe81c873994f99e912fc0c0c4dfa6c0fR253

I'll investigate to see if there's a better way to handle this though.

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