[lit] Bring back --threads option alias

Bring back `--threads` option which was lost in the move of the
command line argument parsing code to cl_arguments.py.  Update docs
since `--workers` is preferred.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@374432 91177308-0d34-0410-b5e6-96231b3b80d8

docs/CommandGuide/lit.rst

@@ -53,7 +53,7 @@ GENERAL OPTIONS
 
  Show the :program:`lit` help message.
 
-.. option:: -j N, --threads=N
+.. option:: -j N, --workers=N
 
  Run ``N`` tests in parallel.  By default, this is automatically chosen to
  match the number of detected available CPUs.

utils/lit/lit/cl_arguments.py

@@ -16,7 +16,7 @@ def parse_args():
             help="Show version and exit",
             action="store_true",
             default=False)
-    parser.add_argument("-j", "--workers",
+    parser.add_argument("-j", "--threads", "--workers",
             dest="numWorkers",
             metavar="N",
             help="Number of workers used for testing",