[cxx-interop] Enable -verify-additional-file tests on Windows (#80694)

These previously failed because they used '/' as the path separator
instead of '\'. While both are accepted on Windows, the latter is
preferred and used to construct new paths, such as the path of imported
header files. This inconsistency led to distinct buffer ID numbers being
created (since paths are not canonicalized in BufferIdentIDMap), leading
-verify to fail to associate the error emitted from the included file
with the error expected in the -verify-additional-file file.

This patch re-enables these tests but uses %{fs-sep} in place of '/', to
ensure the appropriate path separator is used for the platform where the
test is run.

rdar://148928101

Author: j-hui

test/Interop/Cxx/class/access/private-fileid-diagnostics.swift

@@ -1,8 +1,5 @@
 // RUN: split-file %s %t
-// RUN: %target-swift-frontend -typecheck -verify -suppress-remarks %t/some/subdir/file1.swift -verify-additional-file %t/Cxx/include/cxx-header.h -I %t/Cxx/include -cxx-interoperability-mode=default -module-name main
-
-// This test uses -verify-additional-file, which do not work well on Windows:
-// UNSUPPORTED: OS=windows-msvc
+// RUN: %target-swift-frontend -typecheck -verify -suppress-remarks %t%{fs-sep}some%{fs-sep}subdir%{fs-sep}file1.swift -verify-additional-file %t%{fs-sep}Cxx%{fs-sep}include%{fs-sep}cxx-header.h -I %t%{fs-sep}Cxx%{fs-sep}include -cxx-interoperability-mode=default -module-name main
 
 //--- Cxx/include/module.modulemap
 module CxxModule {

test/Interop/Cxx/class/mutability-annotations-typechecker.swift

@@ -1,6 +1,4 @@
-// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -I %S/Inputs %s -enable-experimental-cxx-interop -verify -verify-additional-file %S/Inputs/mutability-annotations.h
-
-// REQUIRES: rdar100876534
+// RUN: %target-swift-frontend(mock-sdk: %clang-importer-sdk) -typecheck -I %S%{fs-sep}Inputs %s -enable-experimental-cxx-interop -verify -verify-additional-file %S%{fs-sep}Inputs%{fs-sep}mutability-annotations.h
 
 import MutabilityAnnotations
 

test/Interop/Cxx/foreign-reference/inheritance-diagnostics.swift

@@ -1,8 +1,5 @@
 // RUN: rm -rf %t
-// RUN: %target-swift-frontend -typecheck -verify -I %S/Inputs  %s -cxx-interoperability-mode=upcoming-swift -verify-additional-file %S/Inputs/inheritance.h -Xcc -Wno-return-type -Xcc -Wno-inaccessible-base
-
-// TODO: Fix this lit test failure on windows rdar://145218056
-// XFAIL: OS=windows-msvc
+// RUN: %target-swift-frontend -typecheck -verify -I %S%{fs-sep}Inputs  %s -cxx-interoperability-mode=upcoming-swift -verify-additional-file %S%{fs-sep}Inputs%{fs-sep}inheritance.h -Xcc -Wno-return-type -Xcc -Wno-inaccessible-base
 
 import Inheritance