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

[Fangrui Song via cfe-commits]

MaskRay wrote:

> I'm not so sure that it is that simple. `-idirafter` and `-isystem-after` have slightly different semantics. They impact ordering and warning suppression. If there are alternative flags that can be used to control those, that might be reasonable.


Do we know the users of `*-windows-itanium` triples? 

Right now `-isystem-after` unimplemented for other targets causes confusion due to [clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-isystem-after-directory](https://clang.llvm.org/docs/ClangCommandLineReference.html#cmdoption-clang-isystem-after-directory).

Does the order matter? In addition, 
https://sourcegraph.com/search?q=context:global+isystem-after+-f:clang/&patternType=keyword&sm=0&filters=%5B%5B%22repo%22%2C%22github.com%2Fmit-ll%2Fgaleed%22%2C%22repo%3A%5Egithub%5C%5C.com%2Fmit-ll%2Fgaleed%24%22%5D%5D
says there is no -isystem-dirafter user.

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