[clang] [Clang] Fix LibTooling doc (PR #90441)

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https://github.com/Sirraide requested changes to this pull request.

Thanks for improving our documentation!

There’s some error handling missing here. The `CommonOptionsParser` constructor would previously just exit, but that is no longer the case here, so we actually need to handle errors now.

https://github.com/llvm/llvm-project/pull/90441