[LLDB][ProcessWindows] Set exit status on instance rather than going through all targets (#159308)

When quitting LLDB on Windows while a process was still running, LLDB
would take unusually long to exit. This was due to a temporary deadlock:

The main thread was destroying the processes. In doing so, it iterated
over the target list:

https://github.com/llvm/llvm-project/blob/88c64f76ed2ca226da99b99f60d316b1519fc7d8/lldb/source/Core/Debugger.cpp#L1095-L1098

This locks the list for the whole iteration. Finalizing the process
would eventually lead to `DebuggerThread::StopDebugging`, which
terminates the process and waits for it to exit:

https://github.com/llvm/llvm-project/blob/88c64f76ed2ca226da99b99f60d316b1519fc7d8/lldb/source/Plugins/Process/Windows/Common/DebuggerThread.cpp#L196
The debugger loop (on a separate thread) would see that the process
exited and call
[`ProcessWindows::OnExitProcess`](https://github.com/llvm/llvm-project/blob/88c64f76ed2ca226da99b99f60d316b1519fc7d8/lldb/source/Plugins/Process/Windows/Common/ProcessWindows.cpp#L656-L673).
This calls the static function
[`Process::SetProcessExitStatus`](https://github.com/llvm/llvm-project/blob/0a7a7d56fc882653335beba0d1f8ea9f26089c22/lldb/source/Target/Process.cpp#L1098-L1126).
This tries to find the process by its ID from the debugger's target
list. Doing so requires locking the list, so the debugger thread would
then be stuck on

https://github.com/llvm/llvm-project/blob/0a7a7d56fc882653335beba0d1f8ea9f26089c22/lldb/source/Target/TargetList.cpp#L403

After 5s, the main thread would give up waiting. So every exit where the
process was still running would be delayed by about 5s.

Since `ProcessWindows` would find itself when calling
`SetProcessExitStatus`, we can call `SetExitStatus` directly.

This can also make some tests run faster. For example, the DIA PDB tests
previously took 15s to run on my PC (24 jobs) and now take 5s. For all
shell tests, the difference isn't that big (only about 3s), because most
don't run into this and the tests run in parallel.