[clang] [clang] Allow extra semicolons inside classes also in C++ pedantic mode. (PR #172209)

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keinflue wrote:

> > Given that the change was made in a core issue - therefore allowing us to backport it, my inclination would be to remove the compatibility warnings all together and just emit a non-extension warning in all language modes. I suspect it would be more useful to users
> > WDYT @shafik @erichkeane ?
> 
> My notes say it was a DR, so back porting makes sense.

If I remove the compatibility warnings and instead use a non-extension warning under `-Wextra-semi`, how should I handle the diagnostic groups for `-Wc++11-extra-semi` and `Wc++98-compat-extra-semi`? Would they remain as groups, but unused by any diagnostic?

https://github.com/llvm/llvm-project/pull/172209