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

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Sirraide wrote:

> I suppose another option is to use a regex in the test to accept either form of canonicalization with a comment that lit tests are sometimes run from network mounts and that's why the test is the way it is.

I mean, at that point we can just delete the test entirely though because testing that ‘either it canonicalises the path or it doesn’t’ feels a bit tautological...

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