[clang] [clang-tools-extra] [Clang] [Diagnostics] Simplify filenames that contain '..' (PR #143520)

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Sirraide wrote:

Ok; that’s done now. Thanks to that clang-tidy test I’ve also managed to figure out how to writer a proper regression test for this patch (only for linux though to be fair; not sure we support Windows-only tests that create files etc., and symlinks on windows require administrator permissions anyway from what I know).

https://github.com/llvm/llvm-project/pull/143520