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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

Thank you for this summary, @luporl !

> I think `-isysroot` should have preference over `DEFAULT_SYSROOT`, but:
> 
> * I don't have much knowledge about the driver.
> * Since this has been the behavior for a long time, I fear that changing it may break some use case.
> * Some refactoring would need to be done, to make it possible to find out if what is being overridden is `DEFAULT_SYSROOT` or `--sysroot`.

My biggest concern would be your 2nd point above -  I'd rather avoid. Like you said - folks might depend on the current behaviour.

> 
>  As for the `isysroot.f90` test, I guess it would need to be removed, unless there is some way to run it only when `DEFAULT_SYSROOT` is not set. 

You could also use `DEFAULT_SYSROOT` to define a LIT "feature" and check for that (via e.g. `REQUIRES`) in the test file. That shouldn't be too difficult.

> But it doesn't seem worth the trouble, as`exec.f90` ends up testing `-isysroot`, when `DEFAULT_SYSROOT` is not specified at config time.

Wouldn't the behaviour of `exec.f90` change depending on whether the user sets `DEFAULT_SYSROOT`? 
 
> My preference is to add support for `--sysroot` on Flang and remove `isysroot.f90`. @kkwli, @banach-space, what are your thoughts on this?

Can you remind me the benefits of using `-isysroot` over `-sysroot` to begin with? I think that switching to `-sysroot` is fine, but I also want to make sure we're not missing anything.

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