Added LLVM_FALLTHROUGH to address warning: this statement may fall through. NFC.

llvm-svn: 304650
diff --git a/clang/lib/AST/ItaniumMangle.cpp b/clang/lib/AST/ItaniumMangle.cpp
index ff55608..c9bb45a 100644
--- a/clang/lib/AST/ItaniumMangle.cpp
+++ b/clang/lib/AST/ItaniumMangle.cpp
@@ -3785,6 +3785,7 @@
     Out << "v1U" << Kind.size() << Kind;
   }
   // Fall through to mangle the cast itself.
+  LLVM_FALLTHROUGH;
       
   case Expr::CStyleCastExprClass:
     mangleCastExpression(E, "cv");