[SYCL] Move __buildin_unique_stable_name dependent funcs to a separate header

This patch moves functions which use `__builtin_unique_stable_name` to a
separate header which should only be included to integration footer.

This was made to get rid of UB when the compiler sees defenition of the
template specialization before seeing a declaration.

Author: dm-vodopyanov

sycl/include/CL/sycl/detail/kernel_desc.hpp

@@ -21,7 +21,8 @@ namespace detail {
 // while the code requires C++17. This code is not supposed to be used by the
 // libsycl.so so it should not be a problem.
 #if __cplusplus > 201402L
-template <auto &S> struct specialization_id_name_generator {};
+// Definition is in spec_const_integration.hpp
+template <auto &S> struct specialization_id_name_generator;
 #endif
 
 #ifndef __SYCL_DEVICE_ONLY__
@@ -58,14 +59,8 @@ template <class Name> struct SpecConstantInfo {
 
 #if __cplusplus >= 201703L
 // Translates SYCL 2020 specialization constant type to its name.
-template <auto &SpecName> const char *get_spec_constant_symbolic_ID() {
-#ifdef SYCL_LANGUAGE_VERSION
-  return __builtin_unique_stable_name(
-      specialization_id_name_generator<SpecName>);
-#else
-  return "";
-#endif
-}
+// Definition is in spec_const_integration.hpp
+template <auto &SpecName> const char *get_spec_constant_symbolic_ID();
 #endif
 
 #ifndef __SYCL_UNNAMED_LAMBDA__

sycl/include/CL/sycl/detail/spec_const_integration.hpp

@@ -0,0 +1,35 @@
+//==---------------------- spec_const_integration.hpp ----------------------==//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#pragma once
+
+// This header file must not be included to any DPC++ headers.
+// This header file should only be included to integration footer.
+
+__SYCL_INLINE_NAMESPACE(cl) {
+namespace sycl {
+namespace detail {
+
+#if __cplusplus >= 201703L
+template <auto &S> struct specialization_id_name_generator {};
+#endif
+
+// Translates SYCL 2020 specialization constant type to its name.
+template <auto &SpecName> const char *get_spec_constant_symbolic_ID() {
+#ifdef SYCL_LANGUAGE_VERSION
+  return __builtin_unique_stable_name(
+      specialization_id_name_generator<SpecName>);
+#else
+  return "";
+#endif
+}
+#endif
+
+} // namespace detail
+} // namespace sycl
+} // __SYCL_INLINE_NAMESPACE(cl)