[SYCL] Address multi_ptr intel/llvm vs spec mismatches. (#10297)

* Deprecate the multi_ptr constructor from accessor(Reg, Void) with
target::local.
* Deprecate the multi_ptr constructor from accessor(Void) and
local_accessor with generic_space.
* Deprecate the multi_ptr constructor from Legacy using local_accessor.
* Add const qualification to the explicit conversion of
multi_ptr<generic_space> to multi_ptr of any other address space.
* Adjust the order of template parameter deduction guides in sync with
the specification.
* Implement support for the constant_space specialization of non-legacy
multi_ptr.

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Co-authored-by: Artur Gainullin <[email protected]>

Author: mmoadeli