[clang] [clang] Add -Wimplicit-fallthrough to -Wextra (PR #97926)

[Aaron Ballman via cfe-commits]

AaronBallman wrote:

> I'm pretty sure I enabled the warning correctly. @AaronBallman are we good to land?

Not quite -- it looks like this change is causing libc++ test failures that were caught by precommit CI:
```
# | /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-jltnz-1/llvm-project/github-pull-requests/build-runtimes/include/c++/v1/regex:3925:5: note: insert '[[clang::fallthrough]];' to silence this warning
# |  3925 |     case 'x':
# |       |     ^
# |       |     [[clang::fallthrough]];
# | /var/lib/buildkite-agent/builds/linux-56-59b8f5d88-jltnz-1/llvm-project/github-pull-requests/build-runtimes/include/c++/v1/regex:3925:5: note: insert 'break;' to avoid fall-through
# |  3925 |     case 'x':
# |       |     ^
# |       |     break;
# | 1 error generated.
```
CC @ldionne 

https://github.com/llvm/llvm-project/pull/97926