[SYCL][Doc][Ext] Initial draft of non-uniform sub-groups proposal (#6169)

Extension proposal to introduce extensions new non-uniform sub-group
types which represent subsets of a sub-group (ballot_group,
cluster_group, tangle_group, opportunistic_group) and new free functions
for creating these. The proposal also introduces new group-related
traits and extensions to the SYCL 2020 execution model, specifically the
nd-range topology and definitions of control flow and concurrent
execution to support these new group types.

This extension is related to the root-group proposal
#6163.

Signed-off-by: Gordon Brown gordon@codeplay.com

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Co-authored-by: John Pennycook <john.pennycook@intel.com>
Co-authored-by: Steffen Larsen <steffen.larsen@intel.com>
Co-authored-by: Greg Lueck <gregory.m.lueck@intel.com>

Author: 4 people