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

[ccotter]

Summary

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

[ccotter]

llvm/include/llvm/XRay/Graph.h

147

I originally noticed this while writing modernize-use-constraints, and implemented https://reviews.llvm.org/D157239 to find any other similar cases.

[kazu]

Thanks!

[ccotter]

Thanks - @kazu would you mind landing this for me as I do not have merge permissions ("Chris Cotter <[e-mail]>").

llvm/include/llvm/XRay/Graph.h

class NeighborEdgeIteratorT
     friend class NeighborEdgeIteratorT<true, IsOut, BaseIt,
                                        const EdgeValueType>;
 
     InternalEdgeMapT *MP;
     VertexIdentifier SI;
 
   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,
                                    const EdgeValueType>(this->I, MP, SI);
     }
 
     NeighborEdgeIteratorT() = default;
     NeighborEdgeIteratorT(BaseIt _I, InternalEdgeMapT *_MP,