Vector component const conversion operators should return references

This is necessary for consistency between the representation of vector types
between host and device, as well as for consistency with the way vector types
behave when defined as structures (as is the case for code ported from CUDA).

Without this patch, this simple test case:

````

const float& pick(float4 const& v)
{ return v.x; }

const volatile float& pick(float4 const volatile& v)
{ return v.x; }

int main() {
	const float4 c = make_float4(0, 1, 2, 3);
	volatile float4 v = make_float4(1, 2, 3, 0);
	std::cout << pick(c) << std::endl;
	std::cout << pick(v) << std::endl;
}
````

will fail to compile when built with the host compiler.

Closes hipamd issue #3

Author: Oblomov

include/hip/amd_detail/amd_hip_vector_types.h

@@ -209,9 +209,15 @@ template <typename __T> struct is_scalar : public integral_constant<bool, __is_s
             Vector data;
 
             __HOST_DEVICE__
-            operator T() const noexcept { return data[idx]; }
+            constexpr operator const T&() const noexcept {
+                return reinterpret_cast<
+                    const T (&)[sizeof(Vector) / sizeof(T)]>(data)[idx];
+            }
             __HOST_DEVICE__
-            operator T() const volatile noexcept { return data[idx]; }
+            constexpr operator const volatile T&() const volatile noexcept {
+                return reinterpret_cast<
+                    const volatile T (&)[sizeof(Vector) / sizeof(T)]>(data)[idx];
+            }
 
 #ifdef __HIP_ENABLE_VECTOR_SCALAR_ACCESSORY_ENUM_CONVERSION__
             // The conversions to enum are fairly ghastly, but unfortunately used in