fix -verify-additional-file path on windows

Author: hnrklssn

test/Interop/C/swiftify-import/import-as-instance-method.swift

@@ -3,7 +3,7 @@
 // RUN: %empty-directory(%t)
 // RUN: split-file %s %t
 
-// RUN: %target-swift-frontend -emit-module -plugin-path %swift-plugin-dir -o %t/Test.swiftmodule -I %t/Inputs -enable-experimental-feature SafeInteropWrappers -strict-memory-safety -verify -verify-additional-file %t/Inputs/instance.h %t/test.swift -I %bridging-path -DVERIFY
+// RUN: %target-swift-frontend -emit-module -plugin-path %swift-plugin-dir -o %t/Test.swiftmodule -I %t%{fs-sep}Inputs -enable-experimental-feature SafeInteropWrappers -strict-memory-safety -verify -verify-additional-file %t%{fs-sep}Inputs%{fs-sep}instance.h %t/test.swift -I %bridging-path -DVERIFY
 // RUN: env SWIFT_BACKTRACE="" %target-swift-frontend -emit-module -plugin-path %swift-plugin-dir -o %t/Test.swiftmodule -I %t/Inputs -enable-experimental-feature SafeInteropWrappers -strict-memory-safety -warnings-as-errors -Xcc -Werror %t/test.swift -dump-macro-expansions -I %bridging-path 2> %t/out.txt
 // RUN: diff --strip-trailing-cr %t/out.txt %t/out.expected