Avoid dangling pointer in variant reflection.

[yuriy0]

The callback will sometimes store the pointer to the class element (fourth arg), e.g. InstanceDataHierarchy::Build

Issue #, if available:

Description of changes: Title says it all. Current code takes a pointer to a temporary variable which is stored and later accessed after it is no longer valid.

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[sconel]

Hi @yuriy0, thanks for contributing to LY we'll take a look at the change.