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

llvm-svn: 229880
diff --git a/llvm/lib/Fuzzer/FuzzerMutate.cpp b/llvm/lib/Fuzzer/FuzzerMutate.cpp
index 60d5623..b28264a 100644
--- a/llvm/lib/Fuzzer/FuzzerMutate.cpp
+++ b/llvm/lib/Fuzzer/FuzzerMutate.cpp
@@ -47,7 +47,7 @@
       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());