Skip test on windows

Author: ChuanqiXu9

clang/test/CIR/Lowering/var-arg-x86_64.c

@@ -1,3 +1,6 @@
+// See the comments in https://github.com/llvm/clangir/pull/1150
+// REQUIRES: !system-windows
+//
 // RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-cir -fno-clangir-call-conv-lowering %s -o %t.cir
 // RUN: FileCheck --input-file=%t.cir %s --check-prefix=CIR
 // RUN: %clang_cc1 -triple x86_64-unknown-linux-gnu -fclangir -emit-llvm -fno-clangir-call-conv-lowering %s -o %t.ll