[os_log] Mark os_log_helper `nounwind`

[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

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@357501 91177308-0d34-0410-b5e6-96231b3b80d8