perf stat: Clear screen only if output file is a tty

The --interval-clear option makes perf stat to clear the terminal at
each interval.  But it doesn't need to clear the screen when it saves
to a file.  Make it fail when it's enabled with the output options.

  $ perf stat -I 1 --interval-clear -o myfile true
  --interval-clear does not work with output

   Usage: perf stat [<options>] [<command>]

      -o, --output <file>   output file name
          --log-fd <n>      log output to fd, instead of stderr
          --interval-clear  clear screen in between new interval

Signed-off-by: Namhyung Kim <[email protected]>
Tested-by: Arnaldo Carvalho de Melo <[email protected]>
Cc: Adrian Hunter <[email protected]>
Cc: Athira Jajeev <[email protected]>
Cc: Ian Rogers <[email protected]>
Cc: Ingo Molnar <[email protected]>
Cc: James Clark <[email protected]>
Cc: Jiri Olsa <[email protected]>
Cc: Kan Liang <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Xing Zhengjun <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Arnaldo Carvalho de Melo <[email protected]>

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