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

[Kelvin Li via cfe-commits]

kkwli wrote:

 > 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?

@luporl Thanks a lot. Your proposed plan looks good to me.



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