[clang-tidy] remove default header-filter for run-clang-tidy

[torbjoernk]

Summary

run-clang-tidy.py was enforcing '-header-filter' parameter with an
unfortunate default value when none was given. Thus, leading to
overwritten clang-tidy configuration (e.g. from .clang-tidy).

This change removes the default value for '-header-filter' resulting in
the default behaviour of clang-tidy itself.

Fixes PR#41426

[hintonda]

I've also had issues with this behavior. It's a bad default, as the PR notes, but is there a way to provide a better one and still respect the config file?

Also, could you add a test for this? Perhaps in clang-tools-extra/test//clang-tidy/run-clang-tidy.cpp.

[hintonda]

After thinking about it, I agree with this change -- there's no way to provide an appropriate default at this level.

So, LGTM, but wait for @alexfh's okay.

[hintonda]

LGTM. If no further comments, I'll commit it for you tomorrow.

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

 
 def get_tidy_invocation(f, clang_tidy_binary, checks, tmpdir, build_path,
                         header_filter, extra_arg, extra_arg_before, quiet,
                         config):
   """Gets a command line for clang-tidy."""
   start = [clang_tidy_binary]
   if header_filter is not None:
     start.append('-header-filter=' + header_filter)
-  else:
-    # Show warnings in all in-project headers by default.
-    start.append('-header-filter=^' + build_path + '/.*')
   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
     # overwrite it.
     (handle, name) = tempfile.mkstemp(suffix='.yaml', dir=tmpdir)
     os.close(handle)