[llvm][llvm-lit] Hide --use-unique-output-file-name from --help

I was too hasty landing an option whose only (potential) use
at this time is LLVM's own CI.

We may be able to remove it before the next branch that would
be the next llvm-lit release outside of llvm, but the timing
may not work out.

So I am hiding the option in case that were to happen.

Author: DavidSpickett

llvm/utils/lit/lit/cl_arguments.py

@@ -175,12 +175,15 @@ def parse_args():
         type=lit.reports.TimeTraceReport,
         help="Write Chrome tracing compatible JSON to the specified file",
     )
+    # This option only exists for the benefit of LLVM's Buildkite CI pipelines.
+    # As soon as it is not needed, it should be removed. Its help text would be:
+    # When enabled, lit will add a unique element to the output file name,
+    # before the extension. For example "results.xml" will become
+    # "results.<something>.xml". The "<something>" is not ordered in any
+    # way and is chosen so that existing files are not overwritten. [Default: Off]
     execution_group.add_argument(
         "--use-unique-output-file-name",
-        help="When enabled, lit will add a unique element to the output file name, "
-        'before the extension. For example "results.xml" will become '
-        '"results.<something>.xml". The "<something>" is not ordered in any '
-        "way and is chosen so that existing files are not overwritten. [Default: Off]",
+        help=argparse.SUPPRESS,
         action="store_true",
     )
     execution_group.add_argument(