Disable -Wimplicit-fallthrough unless src is fixed.

* Current -Wno-error=implicit-fallthrough caused
  clang-tidy assertion fault, see
  https://bugs.llvm.org/show_bug.cgi?id=39312
* We could enable this warning when source file is fixed.

Bug: 117780134
Test: build with WITH_TIDY=1
Change-Id: Ic86d8add52e9e80bd52ac8ca1b6d349f65aaec86