[clang/CodeGenActionTest] Use the platform's path separator for the `DebugInfoCWDCodeGen` test

[akyrtzi]

Summary

Fixes a failure in some Windows configuration.

clang/unittests/Frontend/CodeGenActionTest.cpp

TEST(CodeGenTest, CodeGenFromIRMemBuffer) {
   bool Success = Compiler.ExecuteAction(Action);
   EXPECT_TRUE(Success);
 }
 
 TEST(CodeGenTest, DebugInfoCWDCodeGen) {
   // Check that debug info is accessing the current working directory from the
   // VFS instead of calling \p llvm::sys::fs::current_path() directly.
 
-  auto VFS = std::make_unique<llvm::vfs::InMemoryFileSystem>();
-  VFS->setCurrentWorkingDirectory("/in-memory-fs-cwd");
   auto Sept = llvm::sys::path::get_separator();
+  auto VFS = std::make_unique<llvm::vfs::InMemoryFileSystem>();
+  VFS->setCurrentWorkingDirectory(
+      std::string(llvm::formatv("{0}in-memory-fs-cwd", Sept)));
   std::string TestPath =
       std::string(llvm::formatv("{0}in-memory-fs-cwd{0}test.cpp", Sept));
   VFS->addFile(TestPath, 0, llvm::MemoryBuffer::getMemBuffer("int x;\n"));
 
   auto Invocation = std::make_shared<CompilerInvocation>();
   Invocation->getFrontendOpts().Inputs.push_back(
       FrontendInputFile("test.cpp", Language::CXX));
   Invocation->getFrontendOpts().ProgramAction = EmitLLVM;