gen_compile_commands: use choices for --log_levels option

Use 'choices' to check if the given parameter is valid.

I also simplified the help message because, with 'choices', --help
shows the list of valid parameters:

  --log_level {DEBUG,INFO,WARNING,ERROR,CRITICAL}

I started the help message with a lower case, "the level of log ..."
in order to be consistent with the -h option:

  -h, --help            show this help message and exit

The message "show this help ..." comes from the ArgumentParser library
code, and I do not know how to change it. So, I changed our code.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>

scripts/gen_compile_commands.py

@@ -45,24 +45,18 @@ def parse_arguments():
                    'compile_commands.json in the search directory)')
     parser.add_argument('-o', '--output', type=str, help=output_help)
 
-    log_level_help = ('The level of log messages to produce (one of ' +
-                      ', '.join(_VALID_LOG_LEVELS) + '; defaults to ' +
+    log_level_help = ('the level of log messages to produce (defaults to ' +
                       _DEFAULT_LOG_LEVEL + ')')
-    parser.add_argument(
-        '--log_level', type=str, default=_DEFAULT_LOG_LEVEL,
-        help=log_level_help)
+    parser.add_argument('--log_level', choices=_VALID_LOG_LEVELS,
+                        default=_DEFAULT_LOG_LEVEL, help=log_level_help)
 
     args = parser.parse_args()
 
-    log_level = args.log_level
-    if log_level not in _VALID_LOG_LEVELS:
-        raise ValueError('%s is not a valid log level' % log_level)
-
     directory = args.directory or os.getcwd()
     output = args.output or os.path.join(directory, _DEFAULT_OUTPUT)
     directory = os.path.abspath(directory)
 
-    return log_level, directory, output
+    return args.log_level, directory, output
 
 
 def process_line(root_directory, file_directory, command_prefix, relative_path):