normalize the flag path to make windows CI happy

Author: hyp

lib/Frontend/CompilerInvocation.cpp

@@ -1242,7 +1242,10 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.RequireCxxInteropToImportCxxInteropModule =
       !Args.hasArg(OPT_cxx_interop_disable_requirement_at_import);
   if (const auto *A = Args.getLastArg(OPT_cxx_interop_libcxx_path)) {
-    Opts.cxxInteropCustomLibcxxPath = A->getValue();
+    // Normalize the C++ stdlib path so that it can be compared later.
+    SmallString<256> nativePath;
+    llvm::sys::path::native(A->getValue(), nativePath);
+    Opts.cxxInteropCustomLibcxxPath = nativePath.str();
   }
 
   Opts.VerifyAllSubstitutionMaps |= Args.hasArg(OPT_verify_all_substitution_maps);