[lldb-dap] Use existing lldb::IOObjectSP for DAP IO (NFC). (#128750)
This simplifies the IOStream.cpp implementation by building on top of
the existing lldb::IOObjectSP.
Additionally, this should help ensure clients connected of a
`--connection` specifier properly detect shutdown requests when the
Socket is closed. Previously, the StreamDescriptor was just accessing
the underlying native handle and was not aware of the `Close()` call to
the Socket itself.
This is both nice to have for simplifying the existing code and this
unblocks an upcoming refactor to support the cancel request.
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Co-authored-by: Jonas Devlieghere <jonas@devlieghere.com>