Revert r303375 "LLVM_FALLTHROUGH instead of fall-through comment."

This reverts commit r303375 since it didn't compile.

llvm-svn: 303377
diff --git a/llvm/lib/Demangle/ItaniumDemangle.cpp b/llvm/lib/Demangle/ItaniumDemangle.cpp
index 213a46b..f454ae6 100644
--- a/llvm/lib/Demangle/ItaniumDemangle.cpp
+++ b/llvm/lib/Demangle/ItaniumDemangle.cpp
@@ -1947,7 +1947,7 @@
               break;
             }
           }
-          LLVM_FALLTHROUGH;
+        // falls through
         default:
           // must check for builtin-types before class-enum-types to avoid
           // ambiguities with operator-names