Added type conversion intrinsics

1. Added all type conversion intrinsics
2. NO TESTS have been added. (Will add in next commit)
3. Sanatized code in hip_runtime.h
4. Added passed() to hipTestHalf to make it pass on HIT

Change-Id: I0987963c802fc7ff4d7e07d7b88d86da35da53c9

src/device_util.cpp

@@ -2179,16 +2179,6 @@ __device__ double __hip_fast_dsqrt_rz(double x) {
   return hc::fast_math::sqrt(x);
 }
 
-__device__ double  __longlong_as_double(long long int x)
-{
-  return static_cast<double>(x);
-}
-
-__device__ long long __double_as_longlong(double x)
-{
-  return static_cast<long long>(x);
-}
-
 __device__ void  __threadfence_system(void){
     // no-op
 }