Fix SPQR Solve() when assigning to a Map object

(grafted from 54e576c88af67aa2c8a66d95a104a4ffc5fd1985
)

Eigen/src/SPQRSupport/SuiteSparseQRSupport.h

@@ -147,7 +147,7 @@ class SPQR
       eigen_assert(b.cols()==1 && "This method is for vectors only");
       
       //Compute Q^T * b
-      Dest y; 
+      typename Dest::PlainObject y;
       y = matrixQ().transpose() * b;
         // Solves with the triangular matrix R
       Index rk = this->rank();