fixup: only specifying space is indeed implicit

Author: bogner

clang/test/SemaHLSL/resource_binding_implicit.hlsl

@@ -14,7 +14,7 @@ RWBuffer<int> c;
 // No warning - explicit binding.
 RWBuffer<float> d : register(u0);
 
-// No warning - explicit binding.
+// expected-warning@+1 {{resource has implicit register binding}}
 RWBuffer<float> dd : register(space1);
 
 // No warning - explicit binding.