[SYCL] Prepare deprecated aliases for removal (#22371)
This patch prepares deprecated aliases to be removed in the next
ABI-breaking window (they're not a part of SYCL 2020):
- cl_bool, cl_char etc. (replaced by sycl::opencl::cl_bool,
sycl::opencl::cl_char etc.)
- schar, uchar etc (no new alternative)
- schar2, schar3 ... schar16, longlong2, longlong3 ... longlong16 etc.
(no new alternative)
- cl_char2, cl_char3 ... cl_char16, cl_int2, cl_int3 ... cl_int16 etc.
(no new alternative)
And removes dead code and comments related to SYCL 1.2.1 mode as we do
not support it anymore.
