IO: switch Win32 pipes to PIPE_WAIT and use a sentinel byte

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Cherry-pick changes from main to use a PIPE_WAIT-mode pipe on Windows. Additionally, steal a byte from the buffer to synchronise on as a sentinel byte to check if we are waiting on a client to read from the buffer.

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@swift-ci please test

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• Explanation:
  Switch pipes on Windows to PIPE_WAIT mode to get a blocking pipe semantics. This allows the IOCP loop in the IO monitor to continue to make progress. We take the last byte of the buffer to synchronise the read and write buffer for the pipe.
• Scope:
  This change impacts only pipes on Windows.
• Issues:
  DispatchIO.read spinning while pipe is open on Windows #820
• Original PRs:
  Switch Win32 pipes to PIPE_WAIT with sentinel bufsize #854
• Risk:
  In theory, this change can cause dispatch clients to possibly hang in the scenario that a pipe is given to dispatch with a writer that fills the buffer and no reader that will ever drain the buffer.
• Testing:
  LSP was used with this change and was verified to no longer cause a high CPU utilisation when idle.
• Reviewers:
  @compnerd @rokhinip

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@swift-ci please test

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@swift-ci please test Linux platform