Summary:
Upon `erase`, F14 vector containers destroys the value and erases the item
(i.e., index into the `values_` vector) from the underlying hashtable. However,
the item still needs to be hashable when erasing from the hashtable, so we have
to destroy the value _afterwards_.

This diff fixes the bug.

There are a couple of reasons that this was previous undetected / did not cause
a problem:
  * for POD types, `allocator_traits::destroy` is a no-op.
  * this code path is only hit if the chunk of the destroyed item has hosted
    overflowed items
  * the use was immediately after free
  * our test coverage did not have vector policy + non-SSO string keys

Reviewed By: nbronson

Differential Revision: D7488050

fbshipit-source-id: ea29e875a0c7a39b8deed40a15777a6983438836