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

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Sirraide wrote:

> I don't think an opt-in flag justifies the maintenance burden; I wouldn't expect users to enable that flag by default (honestly, the folks who benefit the most from the canonicalization are likely the same folks who would not think to add a new flag to the command line to get that canonicalization).
> 
> From a Windows perspective, Clang's current behavior is mildly infuriating because it mixes path separators and not all tools handle that well. But it is existing behavior that we've lived with forever, so it's not the end of the world if we continue to not canonicalize paths.
> 
> My weak preference is to consistently canonicalize all paths we want to display to the user on the belief that the canonical path is the most useful form for understanding what's being referenced. But if that's too disruptive to change, I can live with the status quo too.

I mean, another option would be to add a separate flag to disable canonicalisation and enable it by default. I don’t think we want to canonicalise *all* paths though because e.g. `src/foo.cc` is less noisy than e.g. `/home/username/projects/whatever/src/foo.cc` (and we also have a flag for that already; it’s `-fdiagnostics-absolute-paths` or something like that). The point of this change imo (at least this was my original motivation), is to avoid printing excessively convoluted file paths, which is also why I originally added a check to use whichever path ends up being shorter (before or after canonicalisation).

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