Reduce gcc-7 warnings by fall-through comments.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@303365 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Demangle/ItaniumDemangle.cpp

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