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

[Paul T Robinson via cfe-commits]

pogo59 wrote:

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

I think @bd1976bris might have been using that triple for testing the dllimport/visibility intersection. I've lost track and don't know what state that's in now, and whether the triple is still used by us.

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