[SYCL-MLIR][clang] Attach `SYCLKernelObjFunc` to kernel body functions (#12345)
`SemaSYCL` introduces kernels calling user-provided kernel body
functions to avoid OpenCL limitations on kernel arguments. Mark the
original kernel body functions with a new `SYCLKernelObjFunc` attribute
to handle them in codegen.
Signed-off-by: Victor Perez <victor.perez@codeplay.com>