std::is_base_of<A, B> has undefined behaviour if B is not a complete
type. Hence, avoid it and use is_convertible<B*, A*> instead.
This captures exactly the requirement that
  Handle<A> foo() { return produce<Handle<B>>(); }
is valid exactly if
  A* foo() { return produce<B>(); }
is valid.
Also, change some static asserts to enable_ifs in order to allow
overloading a function by different Handle types, which would cause
disambiguity otherwise.

R=tebbi@chromium.org

Change-Id: I60fbdfcfd96c7b216e42819a5b5de3423a2c38d0
Reviewed-on: https://chromium-review.googlesource.com/774841Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Tobias Tebbi <tebbi@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#49440}