bug #1381: fix sparse.diagonal() used as a rvalue.

The problem was that is "sparse" is not const, then sparse.diagonal() must have the
LValueBit flag meaning that sparse.diagonal().coeff(i) must returns a const reference,
const Scalar&. However, sparse::coeff() cannot returns a reference for a non-existing
zero coefficient. The trick is to return a reference to a local member of
evaluator<SparseMatrix>.
(grafted from 296d24be4d
)

Eigen/src/Core/CoreEvaluators.h

@@ -1556,9 +1556,7 @@ struct evaluator<Diagonal<ArgType, DiagIndex> >
   { }
  
   typedef typename XprType::Scalar Scalar;
-  // FIXME having to check whether ArgType is sparse here i not very nice.
-  typedef typename internal::conditional<!internal::is_same<typename ArgType::StorageKind,Sparse>::value,
-                                         typename XprType::CoeffReturnType,Scalar>::type CoeffReturnType;
+  typedef typename XprType::CoeffReturnType CoeffReturnType;
 
   EIGEN_DEVICE_FUNC EIGEN_STRONG_INLINE
   CoeffReturnType coeff(Index row, Index) const

Eigen/src/Core/util/XprHelper.h

@@ -638,7 +638,7 @@ struct plain_constant_type
 template<typename ExpressionType>
 struct is_lvalue
 {
-  enum { value = !bool(is_const<ExpressionType>::value) &&
+  enum { value = (!bool(is_const<ExpressionType>::value)) &&
                  bool(traits<ExpressionType>::Flags & LvalueBit) };
 };