Fix a build break in ArenaAllocator.

Fix a build break in ArenaAllocator.

The toolset used by the internal build (and the ARM build in CI) seems to be a bit
buggy w.r.t. move assignment and deleted assignment operators. In particular,
it attempts to generate an assignment operator for a derived type whose base
type has deleted the corresponding operator and it chooses deleted copy
assignment operators over move assignment operators unless std::move is
used.

[tfs-changeset: 1579941]