[lldb] Fix data race in ThreadList

ThreadSanitizer reports the following issue:

```
  Write of size 8 at 0x00010a70abb0 by thread T3 (mutexes: write M0):
    #0 lldb_private::ThreadList::Update(lldb_private::ThreadList&) ThreadList.cpp:741 (liblldb.18.0.0git.dylib:arm64+0x5dedf4) (BuildId: 9bced2aafa373580ae9d750d9cf79a8f32000000200000000100000000000e00)
    #1 lldb_private::Process::UpdateThreadListIfNeeded() Process.cpp:1212 (liblldb.18.0.0git.dylib:arm64+0x53bbec) (BuildId: 9bced2aafa373580ae9d750d9cf79a8f32000000200000000100000000000e00)

  Previous read of size 8 at 0x00010a70abb0 by main thread (mutexes: write M1):
    #0 lldb_private::ThreadList::GetMutex() const ThreadList.cpp:785 (liblldb.18.0.0git.dylib:arm64+0x5df138) (BuildId: 9bced2aafa373580ae9d750d9cf79a8f32000000200000000100000000000e00)
    #1 lldb_private::ThreadList::DidResume() ThreadList.cpp:656 (liblldb.18.0.0git.dylib:arm64+0x5de5c0) (BuildId: 9bced2aafa373580ae9d750d9cf79a8f32000000200000000100000000000e00)
    #2 lldb_private::Process::PrivateResume() Process.cpp:3130 (liblldb.18.0.0git.dylib:arm64+0x53cd7c) (BuildId: 9bced2aafa373580ae9d750d9cf79a8f32000000200000000100000000000e00)
```

Fix this by only using the mutex in ThreadList and removing the one in
process entirely.

Differential Revision: https://reviews.llvm.org/D158034