IVGCVSW-2056+IVGCVSW-2064 : move ClContextControl to the ClBackend

* add IBackendContext interface
* add ClBackendContext implementation

Change-Id: I13e4d12b73d4c7775069587675276f7cee7d630b
diff --git a/src/backends/cl/CMakeLists.txt b/src/backends/cl/CMakeLists.txt
index d751854..dd2a4a1 100644
--- a/src/backends/cl/CMakeLists.txt
+++ b/src/backends/cl/CMakeLists.txt
@@ -7,6 +7,8 @@
     list(APPEND armnnClBackend_sources
         ClBackend.cpp
         ClBackend.hpp
+        ClBackendContext.cpp
+        ClBackendContext.hpp
         ClBackendId.hpp
         ClContextControl.cpp
         ClContextControl.hpp