tsan: prevent insertion of memset into BenignRaceImpl

Some bots started failing with the following error
after changing Alloc to New. Change it back.

ThreadSanitizer: CHECK failed: ((locked[i].recursion)) == ((0))
 4 __sanitizer::CheckedMutex::CheckNoLocks()
 5 __tsan::ScopedInterceptor::~ScopedInterceptor()
 6 memset
 7 __tsan::New<__tsan::ExpectRace>()
 8 __tsan::AddExpectRace()
 9 BenignRaceImpl()

Differential Revision: https://reviews.llvm.org/D107212