Roll buildtools to clang 12

[dnfield]

Initially this looks like it'll involve:

• Updating buildroot for any new warnings/deprecations
• Potentially rolling the NDK
• Rolling wuffs (avoids some warnings that the current revision has).

Ideally this would be done on by a roller bot.

[dnfield]

We do not need to roll the NDK. I'll file a separate issue for rolling the NDK though, sicne I've done some work for that.

[dnfield]

Fix was reverted because of one more issue with warnings in Xcode. Should reland it shortly.