[lit] Create one output file when --output is specified more than once

The argparse 'append' action concatenates multiple occurrences of an
argument (even when we specify `nargs=1` or `nargs='?'`).  This means
that we create multiple identical output files if the `--output`
argument is given more than once.  This isn't useful and we instead want
this to behave like a standard optional argument: last occurrence wins.

utils/lit/lit/cl_arguments.py

@@ -58,10 +58,7 @@ def parse_args():
             help="Display all commandlines and output",
             action="store_true")
     format_group.add_argument("-o", "--output",
-            dest="reports",
-            action="append",
             type=lit.reports.JsonReport,
-            default=[],
             help="Write test results to the provided path",
             metavar="PATH")
     format_group.add_argument("--no-progress-bar",
@@ -102,10 +99,7 @@ def parse_args():
             help="Don't execute any tests (assume PASS)",
             action="store_true")
     execution_group.add_argument("--xunit-xml-output",
-            dest="reports",
-            action="append",
             type=lit.reports.XunitReport,
-            default=[],
             help="Write XUnit-compatible XML test reports to the specified file")
     execution_group.add_argument("--timeout",
             dest="maxIndividualTestTime",
@@ -190,6 +184,8 @@ def parse_args():
     else:
         opts.shard = None
 
+    opts.reports = filter(None, [opts.output, opts.xunit_xml_output])
+
     return opts