[flang] [clang] [flang][driver] Add support for -isysroot in the frontend (PR #77365)

[Andrzej Warzyński via cfe-commits]

https://github.com/banach-space approved this pull request.

LGTM, this is very thoroughly tested and documented now, thank you 🙏🏻👌🏻

Feel free to ignore my final suggestion.

https://github.com/llvm/llvm-project/pull/77365