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

[Leandro Lupori via cfe-commits]

luporl wrote:

> > Apart from that, the other difference that I'm aware is that on non-Darwin targets -isysroot only sets the sysroot for the includes, while -sysroot also affects the libraries.
> 
> Sounds like in practice it wouldn’t make much difference whether it’s -isysroot or -sysroot?

Right, but adding support for -sysroot too would have 2 main benefits:
- Make it possible to override DEFAULT_SYSROOT.
- Make it possible for other targets (non-Darwin) to set the sysroot.

My plan is the following:
- Try to fix isysroot.f90 test by making DEFAULT_SYSROOT a LIT "feature".
- Keep using -isysroot on Flang's tests, on Darwin, as it seems to be the recommended way to select an SDK.
- Add support for -sysroot.

Does it look good?

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