[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?

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