Build envoy with debug

[nirvanagit]

No description provided.

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[istio-testing]

Hi @nirvanagit. Thanks for your PR.

I'm waiting for a istio member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[lei-tang]

The PR description is currently empty. Can you add to the PR description of 1) why this option needs to be added; 2) pros and cons of adding this option?

[kyessenov]

Also, this will be overwritten by Envoy sync anyways. We have debug builds of Envoy but they are O(Gb) sized. Why do you want this on all builds?