Replace __ANDROID__ with __BIONIC__.

Summary:
None of these checks are specific to Android devices. If libc++ was
used with Bionic on a normal Linux system these checks would still be
needed.

Reviewers: mclow.lists, EricWF

Subscribers: compnerd, tberghammer, danalbert, srhines, cfe-commits

Differential Revision: https://reviews.llvm.org/D24690

llvm-svn: 281921