Warn about uses of `@available` that can't suppress the
-Wunguarded-availability warnings

rdar://32306520

Differential Revision: https://reviews.llvm.org/D33450

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@303761 91177308-0d34-0410-b5e6-96231b3b80d8
(cherry picked from commit aa8caaeedba138d49222e6baae4deaa1a3dfc78e)
5 files changed