Use _LIBCPP_FALLTHROUGH() to avoid warnings about [[gnu::fallthrough]] being unsupported

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@305500 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/cxa_demangle.cpp b/src/cxa_demangle.cpp
index a2c384e..6673931 100644
--- a/src/cxa_demangle.cpp
+++ b/src/cxa_demangle.cpp
@@ -2258,7 +2258,7 @@
                                 break;
                             }
                         }
-                        [[gnu::fallthrough]];
+                        _LIBCPP_FALLTHROUGH();
                     default:
                         // must check for builtin-types before class-enum-types to avoid
                         // ambiguities with operator-names