[libc++] Bump Clang support for Clang 4

It's hard to imagine someone using a recent version of libc++ with a
roughly 3 years old Clang. Since we're not testing libc++ with Clang 3.5
anyway, claiming support for it is somewhat of a lie.

Note that we don't test Clang 4 either, however I have no reason to bump
the requirement beyond Clang 4 at the moment, whereas removing Clang 3.5
allows simplifying the test suite.

Differential Revision: https://reviews.llvm.org/D76618