Fix the total teardown order for ConhostConnection (#1340)

The signal pipe must be terminated first. It is this very termination
that signals to the connected console host that it should begin exiting
in an orderly manner.

We're introducing an indefinite wait (yes, I know: it's not great) for
conhost to exit. **This matches ClosePseudoConsole in
kernelbase/winconpty.**

If it does not exit in an orderly manner, powershell (and perhaps other
.NET CLI applications) may crash immediately after conhost exits.

Fixes #1338.

Author: DHowett

src/cascadia/TerminalConnection/ConhostConnection.cpp

@@ -162,15 +162,22 @@ namespace winrt::Microsoft::Terminal::TerminalConnection::implementation
 
         if (!_closing.exchange(true))
         {
-            _hJob.reset(); // This will terminate the process _piConhost is holding.
-            _piConhost.reset();
-
+            // It is imperative that the signal pipe be closed first; this triggers the
+            // pseudoconsole host's teardown. See PtySignalInputThread.cpp.
+            _signalPipe.reset();
             _inPipe.reset();
             _outPipe.reset();
-            _signalPipe.reset();
 
+            // Tear down our output thread -- now that the output pipe was closed on the
+            // far side, we can run down our local reader.
             WaitForSingleObject(_hOutputThread.get(), INFINITE);
             _hOutputThread.reset();
+
+            // Wait for conhost to terminate.
+            WaitForSingleObject(_piConhost.hProcess, INFINITE);
+
+            _hJob.reset(); // This is a formality.
+            _piConhost.reset();
         }
     }