[lldb] Use LLVM_FALLTHROUGH to avoid a compiler error when building with MSVC.

Author: aganea

lldb/source/Plugins/LanguageRuntime/ObjC/AppleObjCRuntime/AppleObjCRuntimeV2.cpp

@@ -1893,15 +1893,15 @@ AppleObjCRuntimeV2::DynamicClassInfoExtractor::ComputeHelper(
       if (loader->IsFullyInitialized()) {
         switch (exe_ctx.GetTargetRef().GetDynamicClassInfoHelper()) {
         case eDynamicClassInfoHelperAuto:
-          [[clang::fallthrough]];
+          LLVM_FALLTHROUGH;
         case eDynamicClassInfoHelperGetRealizedClassList:
           if (m_runtime.m_has_objc_getRealizedClassList_trylock)
             return DynamicClassInfoExtractor::objc_getRealizedClassList_trylock;
-          [[clang::fallthrough]];
+          LLVM_FALLTHROUGH;
         case eDynamicClassInfoHelperCopyRealizedClassList:
           if (m_runtime.m_has_objc_copyRealizedClassList)
             return DynamicClassInfoExtractor::objc_copyRealizedClassList;
-          [[clang::fallthrough]];
+          LLVM_FALLTHROUGH;
         case eDynamicClassInfoHelperRealizedClassesStruct:
           return DynamicClassInfoExtractor::gdb_objc_realized_classes;
         }