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

[Leandro Lupori via cfe-commits]

luporl wrote:

> If the compiler is built with `DEFAULT_SYSROOT`, the `-isysroot` option is ignored. Is that the expected behavior? My local MacOS build (with `-DDEFAULT_SYSROOT="$(xcrun --show-sdk-path)"`) has `isysroot.f90` failed.

I expected `-isysroot` to override `DEFAULT_SYSROOT`. I'll take a look at that, thanks for reporting it.

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