[CUDA] add wrapper header for libc++'s __utlility/declval.h

Since llvm#116709 more libc++ code relies on std::declval() and it broke some CUDA compilations.

The new wrapper adds GPU-side overloads for the declval() helper functions
which allows it to continue working when used from CUDA sources.

Author: Artem-B

clang/lib/Headers/cuda_wrappers/__utility/declval.h

@@ -0,0 +1,31 @@
+#ifndef __CUDA_WRAPPERS_UTILITY_DECLVAL_H__
+#define __CUDA_WRAPPERS_UTILITY_DECLVAL_H__
+
+#include_next <__utility/declval.h>
+
+// The stuff below is the exact copy of the <__utility/declval.h>,
+// but with __device__ attribute applied to the functions, so it works on a GPU.
+
+_LIBCPP_BEGIN_NAMESPACE_STD
+
+// Suppress deprecation notice for volatile-qualified return type resulting
+// from volatile-qualified types _Tp.
+_LIBCPP_SUPPRESS_DEPRECATED_PUSH
+template <class _Tp>
+__attribute__((device))
+_Tp&& __declval(int);
+template <class _Tp>
+__attribute__((device))
+_Tp __declval(long);
+_LIBCPP_SUPPRESS_DEPRECATED_POP
+
+template <class _Tp>
+__attribute__((device))
+_LIBCPP_HIDE_FROM_ABI decltype(std::__declval<_Tp>(0)) declval() _NOEXCEPT {
+  static_assert(!__is_same(_Tp, _Tp),
+                "std::declval can only be used in an unevaluated context. "
+                "It's likely that your current usage is trying to extract a value from the function.");
+}
+
+_LIBCPP_END_NAMESPACE_STD
+#endif // __CUDA_WRAPPERS_UTILITY_DECLVAL_H__