[ClangScanDeps] Add UNSUPPORTED: system-windows to test P1689.cppm

The revert of
  https://reviews.llvm.org/rG65f9719913eca9c7c72d1abf4eddab767abaebf6

removes a system-windows guard but does not remove the test causing windows
system failures. This patch adds back a guard to prevent further failures.