[SYCL] Update GDB debug info for `sycl::handler` (#21259)
https://github.com/intel/llvm/pull/17319 changed `sycl::handler` to
store raw pointer of `handler_impl` instead of shared pointer. However,
to get handler type in GDB, we still assume `handler->impl` to be a
shared pointer, which is wrong. This PR fixes that.
Refer CMPLRLLVM-72040
