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

[James Y Knight via cfe-commits]

https://github.com/jyknight commented:

The plan of applying only to "internal-isystem"-related flags feels reasonable, although more expansive than I had originally proposed (to apply canonicalization only to finding the GCC installation directories).

I'll note that this does have the potentially-odd result of canonicalizing some of the include paths found from `--sysroot`, without actually canonicalizing sysroot itself. But I don't know of anything concrete that'd be negatively-impacted by that.

So I think this seems OK, modulo some comments.



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