[SYCL][FPGA] Adding function wrapper for function-level loop fusion attributes (#4716)

The function attributes [[intel::loop_fuse(N)]] and
[[intel::loop_fuse_independent(N)]] are being replaced by equivalent
function wrappers by the inclusion of a header file.

These attributes can be accessed now by
sycl::ext::intel::fpga_loop_fuse<N>(F) and
sycl::ext::intel::fpga_loop_fuse_independent<N>(F) for a function or
lambda F.

There is a [lit test](https://github.com/intel/llvm/blob/e9b707dd1653ed74c639b987481bbfe151be2cbd/llvm-spirv/test/transcoding/SPV_INTEL_kernel_attributes/intel_fpga_function_attributes.ll) for the functionality of the loop fusion attributes. The attributes are tested on functions elsewhere (e.g. [here ](https://github.com/intel/llvm/blob/29a1f167060e69d4d90d1e6a15d4998fd4556c4d/clang/test/CodeGenSYCL/loop_fuse_ind_device.cpp) and [here](https://github.com/intel/llvm/blob/29a1f167060e69d4d90d1e6a15d4998fd4556c4d/clang/test/CodeGenSYCL/loop_fuse_device.cpp)).

Author: aaronkintel

sycl/include/sycl/ext/intel/fpga_extensions.hpp

@@ -8,6 +8,7 @@
 
 #pragma once
 #include <sycl/ext/intel/fpga_device_selector.hpp>
+#include <sycl/ext/intel/fpga_loop_fuse.hpp>
 #include <sycl/ext/intel/fpga_lsu.hpp>
 #include <sycl/ext/intel/fpga_reg.hpp>
 #include <sycl/ext/intel/pipes.hpp>

sycl/include/sycl/ext/intel/fpga_loop_fuse.hpp

@@ -0,0 +1,28 @@
+//==--------- fpga_loop_fuse.hpp --- SYCL FPGA Loop Fuse Extension ---------==//
+//
+// 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
+
+__SYCL_INLINE_NAMESPACE(cl) {
+namespace sycl {
+namespace ext {
+namespace intel {
+
+template <int _N = 1, typename _F>
+void fpga_loop_fuse [[intel::loop_fuse(_N)]] (_F f) {
+  f();
+}
+
+template <int _N = 1, typename _F>
+void fpga_loop_fuse_independent [[intel::loop_fuse_independent(_N)]] (_F f) {
+  f();
+}
+
+} // namespace intel
+} // namespace ext
+} // namespace sycl
+} // __SYCL_INLINE_NAMESPACE(cl)