BUG: Fix aliased buffers used in AllReduce().

AllReduce cannot use same buffer for send and receive. This was
causing asserts on Windows dashboards.

Change-Id: I4ee9f32b6db62b89e8557a274b7d79227e41c911
Merge topic '14748_compile_cth_markers_in_by_default' into next

ba184123 BUG: Fix aliased buffers used in AllReduce().