[lldb] Fix data race in ConnectionFileDescriptor

TSan reports the following data race:

  Write of size 4 at 0x000109e0b160 by thread T2 (mutexes: write M0, write M1):
    #0 NativeFile::Close() File.cpp:329
    #1 ConnectionFileDescriptor::Disconnect(lldb_private::Status*) ConnectionFileDescriptorPosix.cpp:232
    rust-lang#2 Communication::Disconnect(lldb_private::Status*) Communication.cpp:61
    rust-lang#3 process_gdb_remote::ProcessGDBRemote::DidExit() ProcessGDBRemote.cpp:1164
    rust-lang#4 Process::SetExitStatus(int, char const*) Process.cpp:1097
    rust-lang#5 process_gdb_remote::ProcessGDBRemote::MonitorDebugserverProcess(...) ProcessGDBRemote.cpp:3387

  Previous read of size 4 at 0x000109e0b160 by main thread (mutexes: write M2):
    #0 NativeFile::IsValid() const File.h:393
    #1 ConnectionFileDescriptor::IsConnected() const ConnectionFileDescriptorPosix.cpp:121
    rust-lang#2 Communication::IsConnected() const Communication.cpp:79
    rust-lang#3 process_gdb_remote::GDBRemoteCommunication::WaitForPacketNoLock(...) GDBRemoteCommunication.cpp:256
    rust-lang#4 process_gdb_remote::GDBRemoteCommunication::WaitForPacketNoLock(...l) GDBRemoteCommunication.cpp:244
    rust-lang#5 process_gdb_remote::GDBRemoteClientBase::SendPacketAndWaitForResponseNoLock(llvm::StringRef, StringExtractorGDBRemote&) GDBRemoteClientBase.cpp:246

The problem is that in WaitForPacketNoLock's run loop, it checks that
the connection is still connected. This races with the
ConnectionFileDescriptor disconnecting. Most (but not all) access to the
IOObject in ConnectionFileDescriptorPosix is already gated by the mutex.
This patch just protects IsConnected in the same way.

Differential revision: https://reviews.llvm.org/D157347

lldb/include/lldb/Host/posix/ConnectionFileDescriptorPosix.h

@@ -131,7 +131,7 @@ class ConnectionFileDescriptor : public Connection {
   lldb::IOObjectSP m_io_sp;
 
   Pipe m_pipe;
-  std::recursive_mutex m_mutex;
+  mutable std::recursive_mutex m_mutex;
   std::atomic<bool> m_shutting_down; // This marks that we are shutting down so
                                      // if we get woken up from
   // BytesAvailable to disconnect, we won't try to read again.

lldb/source/Host/posix/ConnectionFileDescriptorPosix.cpp

@@ -118,6 +118,7 @@ void ConnectionFileDescriptor::CloseCommandPipe() {
 }
 
 bool ConnectionFileDescriptor::IsConnected() const {
+  std::lock_guard<std::recursive_mutex> guard(m_mutex);
   return m_io_sp && m_io_sp->IsValid();
 }