[clang] [Clang] [Driver] Canoncalise `-internal-isystem` include paths (PR #148745)

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Sirraide wrote:

> I was thinking it's also useful for `-###` and `-v` output, crash reports, `-ast-dump` output...

I mean, I feel like those are mainly useful to Clang devs rather than users; I think most people spend much more time looking at diagnostics rather than e.g. the output of `-###`.


https://github.com/llvm/llvm-project/pull/148745