Make folly::allocator_delete take advantage of EBO

Summary: It wasn't.

Test Plan:
I have a case using this that now passes a static_assert
that it's the same size as the default_deleter version.  I didn't add
a static_assert in folly's tests because right now
is_simple_allocator<int,std::allocator<int>>::value is true (I think
because the tested members come from a base class), which breaks easy
test cases.

Reviewed By: andrei.alexandrescu@fb.com

FB internal diff: D833142
Fix some issues in allocator_delete

Summary:
The unique_ptr implementation was getting confused about how
to determine the pointer type, so just tell it.

Facebook: Broke the fxl build; test plan included recompiling it.

Test Plan: Built.

Reviewed By: marcelo.juchem@fb.com

FB internal diff: D834371