[NFC][xray] Fix enable_if usage in XRay.h

The original usage of enable_if does not actually lookup
the nested type called type, so the SFINAE does not kick in.

Differential Revision: https://reviews.llvm.org/D157240

llvm/include/llvm/XRay/Graph.h

@@ -144,7 +144,7 @@ class Graph {
 
   public:
     template <bool IsConstDest,
-              typename = std::enable_if<IsConstDest && !IsConst>>
+              typename = std::enable_if_t<IsConstDest && !IsConst>>
     operator NeighborEdgeIteratorT<IsConstDest, IsOut, BaseIt,
                                    const EdgeValueType>() const {
       return NeighborEdgeIteratorT<IsConstDest, IsOut, BaseIt,