[lldb-dap] Implement runInTerminal for Windows

[SuibianP]

Currently, the named pipe is passed by name and a transient ofstream is constructed at each I/O request. This assumes,

• Blocking semantics: FIFO I/O waits for the other side to connect.
• Buffered semantics: Closing one side does not discard existing data.

The former can be replaced by WaitNamedPipe/ConnectNamedPipe on Win32, but the second cannot be easily worked around. It is also impossible to have another "keep-alive" pipe server instance, as server-client pairs are fixed on connection on Win32 and the client may get connected to it instead of the real one.

Refactor FifoFile[IO] to use an open file handles rather than file name.

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[SuibianP]

The patch currently suffers from a critical issue that I cannot diagnose. Help is much appreciated.

When the debug adapter attempts to attach to the launcher,

llvm-project/lldb/tools/lldb-dap/lldb-dap.cpp

Line 2039 in 4c8c18c

dap.target.Attach(attach_info, error);

it blocks at

llvm-project/lldb/source/Plugins/Process/Windows/Common/ProcessDebugger.cpp

Lines 580 to 581 in 4c8c18c

	if (::WaitForSingleObject(m_session_data->m_initial_stop_event, INFINITE) ==
	WAIT_OBJECT_0) {

although adding the following snippet at the front of RunInTerminalLauncherCommChannel::WaitUntilDebugAdaptorAttaches indicates that the launcher process does get paused and never resumed.

for (int i = 0; !::IsDebuggerPresent(); ++i)
  LLVM_LOGV(log, "Waiting to be attached... {0}", i);
LLVM_LOGV(log, "Attached");

CC @JDevlieghere