[XRay] [compiler-rt] stop writing garbage in naive log records
Turns out sizeof(packed) isn't as strong as we'd hoped. This makes sure
that when we initialize the padding, all 12 bytes will be zero.
Reviewers: dberris, kpw, eizan
Subscribers: delcypher, llvm-commits, #sanitizers
Differential Revision: https://reviews.llvm.org/D42494