[clang-tidy] run-clang-tidy.py: Fix -allow-enabling-analyzer-alpha-checkers always being passed

[jspam]

Summary

The action='store_true' option of argparse.add_argument implicitly
generates a default value of False if the argument is not specified.
Thus, the allow_enabling_alpha_checkers argument of
get_tidy_invocation is never None.

Fixes: bbb7921da97ce03d2933e185a525c4e452b146b0

[1] https://docs.python.org/3/library/argparse.html

[aaron.ballman]

LGTM! Do you need someone to commit on your behalf? If so, please be sure you're fine with the license agreement and let us know what name and email address you would like associated with the commit. Thanks!

[jspam]

Yes please. I am fine with the license agreement, and please use Joachim Priesner <[e-mail]> as the commit author. Thanks!

[aaron.ballman]

I've commit on your behalf as 03ded5497a2f458b6af054fa7bac0da0240e7b7a, thank you for the patch!

clang-tools-extra/clang-tidy/tool/run-clang-tidy.py

def make_absolute(f, directory):
   return os.path.normpath(os.path.join(directory, f))
 
 
 def get_tidy_invocation(f, clang_tidy_binary, checks, tmpdir, build_path,
                         header_filter, allow_enabling_alpha_checkers,
                         extra_arg, extra_arg_before, quiet, config):
   """Gets a command line for clang-tidy."""
   start = [clang_tidy_binary]
-  if allow_enabling_alpha_checkers is not None:
+  if allow_enabling_alpha_checkers:
     start.append('-allow-enabling-analyzer-alpha-checkers')
   if header_filter is not None:
     start.append('-header-filter=' + header_filter)
   if checks:
     start.append('-checks=' + checks)
   if tmpdir is not None:
     start.append('-export-fixes')
     # Get a temporary file. We immediately close the handle so clang-tidy can