[clang] [clang-repl] Improve error message on failed undos (PR #149396)

[David Spickett via cfe-commits]

DavidSpickett wrote:

You have formatting to fix, I suggest you just copy and paste the diff in this one time.

Folks will have different ways to setup clang-format, I use the script https://clang.llvm.org/docs/ClangFormat.html#script-for-patch-reformatting and manually run it (because I am too lazy to integrate it somewhere :) ).

I also add "black" for python formatting, into one giant command:
```
git diff -U0 --no-color --relative HEAD^ | ./clang/tools/clang-format/clang-format-diff.py -p1 -i -binary /home/david.spickett/build-llvm-aarch64/bin/clang-format && darker --revision HEAD~1 $(git diff-tree --no-commit-id --name-only HEAD -r)
```

If you're using an editor, do look into applying the format on save, it's much more convenient.

https://github.com/llvm/llvm-project/pull/149396