[TargetLibraryInfo] Define enumerator for no library function (NFC)

Add a null enumerator do designate no library function.

llvm-svn: 370947

llvm/include/llvm/Analysis/TargetLibraryInfo.h

@@ -30,11 +30,12 @@ struct VecDesc {
   unsigned VectorizationFactor;
 };
 
-  enum LibFunc {
+  enum LibFunc : unsigned {
 #define TLI_DEFINE_ENUM
 #include "llvm/Analysis/TargetLibraryInfo.def"
 
-    NumLibFuncs
+    NumLibFuncs,
+    NotLibFunc
   };
 
 /// Implementation of the target library information.

llvm/lib/Analysis/TargetLibraryInfo.cpp

@@ -1478,6 +1478,7 @@ bool TargetLibraryInfoImpl::isValidProtoForLibFunc(const FunctionType &FTy,
       return false;
   }
   case LibFunc::NumLibFuncs:
+  case LibFunc::NotLibFunc:
     break;
   }