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

This reverts commit r303375 since it didn't compile.

llvm-svn: 303377

llvm/lib/Demangle/ItaniumDemangle.cpp

@@ -1947,7 +1947,7 @@ static const char *parse_type(const char *first, const char *last, C &db) {
               break;
             }
           }
-          LLVM_FALLTHROUGH;
+        // falls through
         default:
           // must check for builtin-types before class-enum-types to avoid
           // ambiguities with operator-names