Remove `getRawSyclObjImpl()` from DPC++ headers.

[bader]

          It seems `getRawSyclObjImpl()` and `getSyclObjImpl().get()` are not identical right now in terms of final asm, see https://godbolt.org/z/6sacE3hrc .

Suggest changing getSyclObjImpl to return by ref, then they get identical.

Originally posted by @romanovvlad in #10458 (comment)