really fix the LDLt, at the expense of letting isPositiveDefinite() always return true (it was fundamentally broken anyway, especially as in 2.0 we don't even pivot at all).

also, fix compilation

test/cholesky.cpp

@@ -100,6 +100,7 @@ template<typename MatrixType> void cholesky(const MatrixType& m)
     VERIFY_IS_APPROX(symm * matX, matB);
   }
 
+#if 0 // cholesky is not rank-revealing anyway
   // test isPositiveDefinite on non definite matrix
   if (rows>4)
   {
@@ -109,6 +110,7 @@ template<typename MatrixType> void cholesky(const MatrixType& m)
     LDLT<SquareMatrixType> cholnosqrt(symm);
     VERIFY(!cholnosqrt.isPositiveDefinite());
   }
+#endif
 }
 
 void test_cholesky()