[clang] [Driver,CrossWindows] Remove -isystem-after (PR #84121)

[Fangrui Song via cfe-commits]

MaskRay wrote:

> > Facebook was using it at one point (can't say if they are currently), I know that Sony had some interested users as well.
> > Yes, the order matters - that was the reason that the flag was implemented in the first place.
> 
> Cc @smeenai @pogo59 on whether this option is used.
> 
> In addition, is `*-windows-itanium` used?

Most updates to CrossWindows are actually updates to other `ToolChain`s and the author happened to notice that CrossWindows needs an update as well.
There has been no specific update to CrossWindows, since at least 2017.

The 2017 change to CrossWindows (https://reviews.llvm.org/D36364) was really for mingw.

https://github.com/llvm/llvm-project/pull/84121