Update Windows Installation for Clang Build

[mraunak]

This pull request will update the https://www.tensorflow.org/install/source_windows with the steps and commands to set up the LLVM/CLANG compiler correctly and guide users on how to build TensorFlow on the Windows Platform using the CLANG compiler.

[github-actions]

Preview

Preview and run these notebook edits with Google Colab:

• site/en/guide/migrate/evaluator.ipynb
• site/en/guide/sparse_tensor.ipynb
• site/en/guide/tf_numpy_type_promotion.ipynb
• site/en/hub/tutorials/s3gan_generation_with_tf_hub.ipynb
• site/en/hub/tutorials/wiki40b_lm.ipynb
• site/en/r1/guide/autograph.ipynb
• site/en/r1/guide/distribute_strategy.ipynb
• site/en/r1/tutorials/representation/unicode.ipynb
• site/en/tutorials/images/data_augmentation.ipynb
• site/en/tutorials/images/transfer_learning.ipynb
• site/en/tutorials/keras/text_classification.ipynb
• site/en/tutorials/quickstart/advanced.ipynb
• site/en/tutorials/structured_data/time_series.ipynb

Rendered notebook diffs available on ReviewNB.com.

Format and style

Use the TensorFlow docs notebook tools to format for consistent source diffs and lint for style:

$ python3 -m pip install -U --user git+https://github.com/tensorflow/docs

$ python3 -m tensorflow_docs.tools.nbfmt notebook.ipynb

$ python3 -m tensorflow_docs.tools.nblint --arg=repo:tensorflow/docs notebook.ipynb

If commits are added to the pull request, synchronize your local branch: git pull origin mraunak/tf_win_clang

[haifeng-jin]

I do not have enough expertise to review this. So leaving it to the rest of the reviewers.