[libc++] Add a test to pin down the set of transitive public includes

[libc++] Add a test to pin down the set of transitive public includes

A situation that happens fairly often in libc++ is that we remove some
transitive includes in a header (either purposefully or not) and that
ends up breaking users. Of course, we want to be able to remove our
transitive includes, however it's also good to have a grip on that
to know which commit changed what and when. Furthermore, it's good
to accumulate include removals for a couple of releases to avoid
breaking users at every release for this reason.

This commit adds a test that should break whenever we remove an
include. Hence, it should allow us to track which headers include
which other headers transitively, giving us a traceable way to
remove headers.

Differential Revision: https://reviews.llvm.org/D128236