[opt-viewer] opt-viewer.py takes -o argument

Summary:
Change how the output directory is specified when invoking
opt-viewer.py, from `opt-viewer.py yaml_file_one yaml_file_two output_dir` to
`opt-viewer.py -o output_dir yaml_file_one yaml_file_two`.

This makes it easier to pipe the results of another command into
opt-viewer.py. For example:

```
find . -name "*.yaml" -print | xargs /path/to/opt-viewer.py -o html
```

Reviewers: anemet, davidxl

Reviewed By: anemet

Subscribers: fhahn, llvm-commits

Differential Revision: https://reviews.llvm.org/D34711

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@306694 91177308-0d34-0410-b5e6-96231b3b80d8

utils/opt-viewer/opt-viewer.py

@@ -202,7 +202,13 @@ def generate_report(pmap, all_remarks, file_remarks, source_dir, output_dir, sho
 if __name__ == '__main__':
     parser = argparse.ArgumentParser(description=desc)
     parser.add_argument('yaml_files', nargs='+')
-    parser.add_argument('output_dir')
+    parser.add_argument(
+        '--output-dir',
+        '-o',
+        default='html',
+        help='Path to a directory where generated HTML files will be output. '
+             'If the directory does not already exist, it will be created. '
+             '"%(default)s" by default.')
     parser.add_argument(
         '--jobs',
         '-j',