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

[Andrzej Warzyński via cfe-commits]

banach-space wrote:

> > 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.
> 
> It seems to me that Apple prefers to use `-isysroot` to select the SDK: https://developer.apple.com/library/archive/documentation/DeveloperTools/Conceptual/cross_development/Configuring/configuring.html#//apple_ref/doc/uid/10000163i-CH1-SW1
> 
> Also, when grepping through CMake generated files (for llvm), I saw that they use `-isysroot` when compiling sources with the clang binary built from source.
> 
> 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.

Alignment with the official guidelines would be nice. If possible :)

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