[SYCLomatic] add null_type implementation (#528)

Signed-off-by: Dan Hoeflinger <[email protected]>

Author: danhoeflinger

clang/runtime/dpct-rt/include/dpl_extras/functional.h.inc

@@ -56,6 +56,12 @@
 
 namespace dpct {
 
+// DPCT_LABEL_BEGIN|null_type|dpct
+// DPCT_DEPENDENCY_EMPTY
+// DPCT_CODE
+struct null_type {};
+// DPCT_LABEL_END
+
 namespace internal {
 
 // DPCT_LABEL_BEGIN|enable_if_execution_policy|dpct::internal

clang/test/dpct/helper_files_ref/include/dpl_extras/functional.h

@@ -22,6 +22,8 @@
 
 namespace dpct {
 
+struct null_type {};
+
 namespace internal {
 
 template <class _ExecPolicy, class _T>