[libcxx] Suppress unused warning on apple.

Summary:
This warning is already suppressed on non-apple platforms, so
this change just suppresses it on apple as well.

Reviewers: EricWF, lichray

Reviewed By: lichray

Subscribers: cfe-commits

Differential Revision: https://reviews.llvm.org/D41248

git-svn-id: https://llvm.org/svn/llvm-project/libcxx/trunk@321435 91177308-0d34-0410-b5e6-96231b3b80d8
1 file changed