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

[Aaron Ballman via cfe-commits]

AaronBallman 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 `-###`.

Proportionally speaking, definitely agreed! But that doesn't mean folks don't write scripts that process `-###` or read compile_commands.json, etc.

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