cxa_demangle: fix -Wimplicit-fallthrough for GCC:7

Use the C++11 (formalised in C++17) tag to indicate a fallthrough in the
switch case.  Silences a -Wimplicit-fallthrough warning with gcc:7

llvm-svn: 305173

libcxxabi/src/cxa_demangle.cpp

@@ -2258,7 +2258,7 @@ parse_type(const char* first, const char* last, C& db)
                                 break;
                             }
                         }
-                        // drop through
+                        [[gnu::fallthrough]];
                     default:
                         // must check for builtin-types before class-enum-types to avoid
                         // ambiguities with operator-names