Update setup and build instructions.

[secure-sw-dev-bot]

This issue was copied from checkedc/checkedc-clang#2

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Fix a few typos in Setup-and-Build.md. Add a missing
hyperlink for how to update to newer versions of the
LLVM/clang sources.

[secure-sw-dev-bot]

Comment from @msftclas:

Hi @dtarditi, I'm your friendly neighborhood Microsoft Pull Request Bot (You can call me MSBOT). Thanks for your contribution!

It looks like you're a Microsoft contributor (David Tarditi). If you're full-time, we DON'T require a Contribution License Agreement. If you are a vendor, please DO sign the electronic Contribution License Agreement. It will take 2 minutes and there's no faxing! https://cla.microsoft.com.

TTYL, MSBOT;