Allow implicit-fallthrough warnings locally. DO NOT MERGE.

It will be a global error by default.
This change is not needed for goog/master.

Test: make checkbuild
Bug: 112564944
Change-Id: Ie90ad1f59f6922da6cecd4a0ac136b1e42b6d482
Exempt-From-Owner-Approval: do not block on new warnings
1 file changed