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

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Sirraide wrote:

> Windows doesn't have `/tmp` for example. I don't think we have any [substitutions](https://llvm.org/docs/CommandGuide/lit.html#substitutions) for getting the temp directory.

Ah, I meant non-windows systems (I thought `REQUIRES: shell` already meant non-windows because the test uses quite a few unix commands but I might be wrong). But also, doesn’t windows have like `%TEMP%` or something? If that works we could probably ‘just’ add a substitution for that (I’m not an expert on anything cross-platform though so this might still be an issue for some platforms).

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