tsan: add support for passing file descriptors over UNIX domain sockets

tsan was missing new fd's arrived from recvmsg(),
and thus was reporting false positives due to missed synchronization on the fd's



git-svn-id: https://llvm.org/svn/llvm-project/compiler-rt/trunk@195914 91177308-0d34-0410-b5e6-96231b3b80d8