Include <sstream> explicitly, and don't rely on the implicit include via <complex>.

This implicit dependency does no longer exist in a recent llbm release (sha 78be61871704).

Eigen/Core

@@ -72,6 +72,7 @@
 #include <cmath>
 #include <cassert>
 #include <functional>
+#include <sstream>
 #ifndef EIGEN_NO_IO
   #include <iosfwd>
 #endif

Eigen/src/Core/arch/Default/Half.h

@@ -42,6 +42,7 @@
 #define EIGEN_EXPLICIT_CAST(tgt_type) operator tgt_type()
 #endif
 
+#include <sstream>
 
 namespace Eigen {