remove using namespace std and use std namespace explicitly (#2125)

Removes `using namespace std` and adds `std::` explicitly instead, which
is usually on calls to `min`.

This is generally best practice, and it also might be helpful when there
are the same function names in the std namespace and in the global
namespace (e.g. #1833).

test_conformance/api/test_queue_hint.cpp

@@ -19,11 +19,6 @@
 #include <sstream>
 #include <string>
 
-using namespace std;
-/*
-
-*/
-
 const char *queue_hint_test_kernel[] = {
 "__kernel void vec_cpy(__global int *src, __global int *dst)\n"
 "{\n"