Enforce clang for Windows builds

[jathu]

Summary

• We currently only support MSVC+clang, so let's enforce it
• Use clang in CI after Support clang in MSVC test-infra#6654

Test plan

CI

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/executorch/11159

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