[os_log] Mark os_log_helper `nounwind`

Allow the optimizer to remove unnecessary EH cleanups surrounding calls
to os_log_helper, to save some code size.

As a follow-up, it might be worthwhile to add a BasicNoexcept exception
spec to os_log_helper, and to then teach CGCall to emit direct calls for
callees which can't throw. This could save some compile-time.

Differential Revision: https://reviews.llvm.org/D60108

llvm-svn: 357501