device_selector: implement select_device.

This allows the CTS test_device_selector test to compile, run
and pass in some circumstances.

include/CL/sycl/device_selector.hpp

@@ -9,9 +9,6 @@
     License. See LICENSE.TXT for details.
 */
 
-#include "CL/sycl/detail/unimplemented.hpp"
-#include "CL/sycl/device.hpp"
-
 namespace cl {
 namespace sycl {
 
@@ -25,17 +22,15 @@ namespace sycl {
 */
 class device_selector {
 
+  device select_device(vector_class<platform> platforms) const;
 public:
 
   /** Returns a selected device using the functor operator defined in
       sub-classes operator()(const device &dev)
 
       \todo Remove this from specification
   */
-  void /* device */ select_device() const {
-    //    return {};
-  }
-
+  device select_device() const;
 
   /**  This pure virtual operator allows the customization of device
        selection.

include/CL/sycl/device_selector/detail/device_selector_tail.hpp

@@ -23,7 +23,32 @@ namespace sycl {
 /** \addtogroup execution Platforms, contexts, devices and queues
     @{
 */
+inline device device_selector::select_device(vector_class<platform> platforms) const {
+  int best_id = -1;
+  int best_score = -1;
+  int i = 0;
+  device best_device;
+
+  for (auto &platform : platforms) {
+    for (auto &dev : platform.get_devices()) {
+      int score = operator()(dev);
+      if (score > best_score) {
+        best_id = 1;
+        best_device = dev;
+        best_score = score;
+      }
+      ++i;
+    }
+  }
+  if (best_id == -1)
+    throw cl::sycl::runtime_error("Could not find device");
 
+  return best_device;
+}
+
+inline device device_selector::select_device() const  {
+  return select_device(platform::get_platforms());
+}
 
 /** A device selector by device_type