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

• 📄 Preview Python docs built from this PR

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❌ 3 New Failures

As of commit 6a26082 with merge base d8ac866 ():

NEW FAILURES - The following jobs have failed:

• Build Presets / windows (llm) / build (gh)
• Build Presets / windows (pybind) / build (gh)
  Process completed with exit code 1.
• pull / unittest / linux / linux-job (gh)
  examples/models/llama/tests/test_export_llama_lib.py::ExportLlamaLibTest::test_has_expected_ops_and_op_counts

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