IVGCVSW-1952 : add BackendId class to prepare for the replacement of Compute enum

!armnn:152674

Change-Id: I1bcdfdfbfb73e502d58f35717e2558e24651013c
diff --git a/src/backends/cl/ClBackend.hpp b/src/backends/cl/ClBackend.hpp
index c43b6a6..b927db4 100644
--- a/src/backends/cl/ClBackend.hpp
+++ b/src/backends/cl/ClBackend.hpp
@@ -16,7 +16,7 @@
     ClBackend()  = default;
     ~ClBackend() = default;
 
-    const std::string& GetId() const override;
+    const BackendId& GetId() const override;
 
     const ILayerSupport& GetLayerSupport() const override;