[fuzzer] properly annotate fallthrough, add one more entry to FAQ

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@229880 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Fuzzer/FuzzerMutate.cpp

@@ -47,7 +47,7 @@ void Mutate(Unit *U, size_t MaxLen) {
       U->erase(U->begin() + rand() % U->size());
       break;
     }
-    // Fallthrough
+    [[clang::fallthrough]];
   case 1:
     if (U->size() < MaxLen) {
       U->insert(U->begin() + rand() % U->size(), RandCh());