[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.

llvm-svn: 374432
diff --git a/llvm/docs/CommandGuide/lit.rst b/llvm/docs/CommandGuide/lit.rst
index 7bb499b..ff70a21 100644
--- a/llvm/docs/CommandGuide/lit.rst
+++ b/llvm/docs/CommandGuide/lit.rst
@@ -53,7 +53,7 @@
 
  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.