[SYCL][COMPAT] kernel_function and kernel_library constexpr constructors (#13932)

Updated classes in `sycl/include/syclcompat/kernel.hpp` to have
constexpr constructors and align to SYCLomatic's latest `kernel.hpp`
header.

Signed-off-by: Alberto Cabrera <[email protected]>

Author: Alcpz

sycl/doc/syclcompat/README.md

@@ -1426,8 +1426,8 @@ static void get_kernel_function_info(kernel_function_info *kernel_info,
 static kernel_function_info get_kernel_function_info(const void *function);
 
 class kernel_library {
-  kernel_library();
-  kernel_library(void *ptr);
+  constexpr kernel_library();
+  constexpr kernel_library(void *ptr);
   operator void *() const;
 };
 
@@ -1436,8 +1436,8 @@ static kernel_library load_kernel_library_mem(char const *const image);
 static void unload_kernel_library(const kernel_library &library);
 
 class kernel_function {
-    kernel_function();
-    kernel_function(kernel_functor ptr);
+    constexpr kernel_function();
+    constexpr kernel_function(kernel_functor ptr);
     operator void *() const;
     void operator()(sycl::queue &q, const sycl::nd_range<3> &range,
                     unsigned int local_mem_size, void **args, void **extra);

sycl/include/syclcompat/kernel.hpp

@@ -340,8 +340,8 @@ class path_lib_record {
 
 class kernel_library {
 public:
-  kernel_library() : ptr{nullptr} {}
-  kernel_library(void *ptr) : ptr{ptr} {}
+  constexpr kernel_library() : ptr{nullptr} {}
+  constexpr kernel_library(void *ptr) : ptr{ptr} {}
 
   operator void *() const { return ptr; }
 
@@ -415,8 +415,8 @@ static inline void unload_kernel_library(const kernel_library &library) {
 
 class kernel_function {
 public:
-  kernel_function() : ptr{nullptr} {}
-  kernel_function(kernel_functor ptr) : ptr{ptr} {}
+  constexpr kernel_function() : ptr{nullptr} {}
+  constexpr kernel_function(kernel_functor ptr) : ptr{ptr} {}
 
   operator void *() const { return ((void *)ptr); }