When we're dealing with virtual (memory) threads created by the OS
plugins, there's no guarantee that the real thread and the backing
thread share a protocol ID. Instead, we should iterate over the memory
threads to find the virtual thread that is backed by the current real
thread.
Differential revision: https://reviews.llvm.org/D45497
rdar://36485830
The original revision (r329891) was reverted because the associated
tests ran into a deadlock on the Linux bots. That problem was resolved
by r330002.
llvm-svn: 330005
8db3f7ede6