[clang] Give the __COUNTER__ pedantic diagnostic a group (PR #218669)

[Corentin Jabot via cfe-commits]

cor3ntin wrote:

@AaronBallman You can make a consistency argument, however saying the diagnostic is useful because not all implementation support it is a bit strong. You have to go back to GCC 4.1 for that to be an issue.
It might be a valid argument for C implementations, but not C++ https://compiler-explorer.com/z/5W9EjdWWP

https://github.com/llvm/llvm-project/pull/218669