heap_vector_types: unbreak heapify() with containers with weird references
heap_vector_types: unbreak heapify() with containers with weird references

Summary: See inline comment. In brief, `Container::reference` not being a reference makes `auto` behave unexpectedly.

Reviewed By: Gownta

Differential Revision: D40496221

fbshipit-source-id: 85f964d3a12f5c596568866de44b213dda22ee54

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly