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

[David Spickett via cfe-commits]

DavidSpickett wrote:

> I am able to build clang, compile the project, and I am currently running the "ninja check-clang-unit" tests. Although it does take an incredibly long time ( 45 minutes to an hour) but that's off topic

llvm/clang is not a light build at the best of times, but there are some things you can try - https://llvm.org/docs/GettingStarted.html#common-problems

If you do pick up another issue, I can give you some tailored suggestions for the specific task. For instance here, "check-clang" is runs all clang checks but I happened to know that the test you're changing is what we call a "unit test" and has it's own smaller target.

> some of the commits in this PR are me updating the branch with changes to the main branch. I think its okay because this whole thing will get squashed at the end but just checking

That's right. In fact I think those "merge" commits are preferred during review, versus rebasing, because GitHub likes to hide review state if you do the latter. It's also fine to not update the PR branch until review has concluded.

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