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

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Sirraide wrote:

> do so by default, but disabled by "-no-canonical-prefix"

So just to be clear, while there might be a better place to canonicalise the paths (we can’t treat include paths the same as the resource directory because that’s canonicalised in `clang_main`), the main thing you care about is being able to turn off the canonicalisation entirely with `-no-canonical-prefix`, is that right?

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