[NFCI][SYCL] Make get_group_id to return by reference

It will enforce address space to the return value helping to
avoid invalid AS cast from local to private for sret argument
that would appear after llvm/llvm-project#128166
and the related work.

Signed-off-by: Sidorov, Dmitry <[email protected]>

Author: MrSidims

sycl/include/sycl/group.hpp

@@ -120,7 +120,7 @@ template <int Dimensions = 1> class __SYCL_TYPE(group) group {
   __SYCL2020_DEPRECATED("use sycl::group::get_group_id() instead")
   size_t get_id(int dimension) const { return index[dimension]; }
 
-  id<Dimensions> get_group_id() const { return index; }
+  id<Dimensions> &get_group_id() { return index; }
 
   size_t get_group_id(int dimension) const { return index[dimension]; }