Teach Clang about syslog format attribute

Index: clang/include/clang/Sema/Sema.h
--- clang/include/clang/Sema/Sema.h.orig
+++ clang/include/clang/Sema/Sema.h
@@ -12648,6 +12648,7 @@ class Sema final { (public)
     FST_FreeBSDKPrintf,
     FST_OSTrace,
     FST_OSLog,
+    FST_Syslog,
     FST_Unknown
   };
   static FormatStringType GetFormatStringType(const FormatAttr *Format);
