[perl #40712] [TODO] Tcl - check for open channel in src/builtin/flush.pir

[Paul Cochrane]

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From the comment in the code, it seems that ParrotIO.pmc can't yet
handle checking if a channel is open. Once this is the case, make the
check explicit in languages/tcl/src/builtin/flush.pir

This ticket is in response to cage task #39704.

[Paul Cochrane]

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