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.cpp b/src/backends/cl/ClBackend.cpp
index d185c15..840da8b 100644
--- a/src/backends/cl/ClBackend.cpp
+++ b/src/backends/cl/ClBackend.cpp
@@ -12,7 +12,7 @@
 
 namespace
 {
-static const std::string s_Id = "GpuAcc";
+static const BackendId s_Id{"GpuAcc"};
 
 static BackendRegistry::Helper g_RegisterHelper{
     s_Id,
@@ -24,7 +24,7 @@
 
 }
 
-const std::string& ClBackend::GetId() const
+const BackendId& ClBackend::GetId() const
 {
     return s_Id;
 }