Allow implicit-fallthrough warnings locally.

It will be a global error by default.
These warnings will be fixed in new refactored code.

Bug: 112564944
Bug: 116020782
Test: make checkbuild
Change-Id: I67f1170f3f7502a4243d8a5641eb471317b42cf4
Exempt-From-Owner-Approval: do not block on new warnings