Have attribute 'objc_precise_lifetime' suppress -Wunused.

Fixes <rdar://problem/15596883>

In ARC, __attribute__((objc_precise_lifetime)) guarantees that the
object stored in it will survive to the end of the variable's formal
lifetime.  It is therefore useful even if it completely unused.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@198888 91177308-0d34-0410-b5e6-96231b3b80d8