* Make `_Modify_annotation(0)` a safe no-op.
  + Add a comment explaining why.

* Undo logic changes in `assign(n, val)` now that `_ASAN_VECTOR_MODIFY`
  is always safe.
  + There are no potential gains from calling `clear()` as this function
    begins by unconditionally calling `_My_data._Orphan_all();` and is
    required to do so by the Standard.

* Comment `_ASAN_VECTOR_MODIFY` with `// negative when destroying elements`
  when subtracting pointers, making it clear that the order of operands
  is intentional.

* Similarly, undo logic changes in `assign(i, j)` and `assign(il)`.
  They are also required to invalidate all iterators, so they can't
  benefit from `clear()`'s optimization here.

* Ditto for `operator=(il)`. (The Standard isn't absolutely clear here,
  but behaving identically to `assign(il)` seems obviously good.)

* Extend the `clear()` optimization to handle all empty vectors
  (zero size), not just zero capacity.
  + Add a comment explaining why, and citing the Standard for why this
    is allowed.

* Similarly undo logic changes in `_Move_assign_unequal_alloc()`.

* Add test coverage for calling `assign()` with an empty range of
  input iterators.