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

[via cfe-commits]

keinflue wrote:

> > 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?
> 
> Can we alias those diagnostic groups to `-Wextra-semi`? I feel like they might still be useful for people who try to be compatible w/ older compilers (where this hasn’t been backported)

That might be ok. A current user of `-Wc++11-extra-semi` or `-Wc++98-compat-extra-semi` would then, I think, get additional warnings for single `;` after member function definitions (which was always well-formed) and for extra `;` in instance variable lists if they use Objective-C++, but not more. Otherwise behavior would stay the same for them and they would not have to update their flags. And I don't think a style of putting `;` after member function definitions is common.

Before I replace the current patch with this approach, maybe some others input might be a good idea?

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