Remove unnecessary fallthrough annotation after unreachable

Remove unnecessary fallthrough annotation after unreachable

Clang's -Wimplicit-fallthrough implementation warns on this. I built
clang with GCC 7.3 in +asserts and -asserts mode, and GCC doesn't warn
on this in either configuration. I think it is unnecessary. I separated
it from the large mechanical patch (https://reviews.llvm.org/D53950) in
case I am wrong and it has to be reverted.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@345876 91177308-0d34-0410-b5e6-96231b3b80d8