Copy MDC-iOS' warnings.#3

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ajsecord wants to merge 3 commits intomaterial-foundation:developfrom

ajsecord:add-warnings

Commits on Nov 16, 2017

Copy https://github.com/material-components/material-components-ios/pull/2456 until they can be rationalized.
ajsecord
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Review suggestions.
ajsecord
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Changed -Wshadow-all back to -Wshadow, since -Wshadow-all doesn't seem to be supported by Kokoro?
ajsecord
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