Add missing constructors for Scalar_accessor class. (#1792)

[ROCm/hip commit: fae85cf6d2]

projects/hip/include/hip/hcc_detail/hip_vector_types.h

@@ -105,6 +105,14 @@ THE SOFTWARE.
             // Idea from https://t0rakka.silvrback.com/simd-scalar-accessor
             Vector data;
 
+            __host__ __device__
+            Scalar_accessor() = default;
+
+            __host__ __device__
+            Scalar_accessor(T x) {
+                data[idx] = x;
+            }
+
             __host__ __device__
             operator T() const noexcept { return data[idx]; }
             __host__ __device__