clang-tidy helper script, with clang static analyzer included.

This CL makes it easy to invoke the tool for single cc files,
without having to install or configure anything.
It boils down to:
% tools_webrtc/clang_tidy.py path/to/file.cc [clang-tidy-args...]
If any issue is encountered, a colored diagnostic will be printed.
By default, it also includes checks from clang analyzer.

Warning! This is linux-only, and uses an old version of
clang-tidy based on LLVM 5.0. USE WITH CARE.

bug: webrtc:8793
Change-Id: I8964f2b939408326cc349c5f0ac0dfcff2da24c5
Reviewed-on: https://webrtc-review.googlesource.com/c/120221
Commit-Queue: Yves Gerey <yvesg@webrtc.org>
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Mirko Bonadei <mbonadei@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#26858}
diff --git a/PRESUBMIT.py b/PRESUBMIT.py
index 8c75942..bc4eaf2 100755
--- a/PRESUBMIT.py
+++ b/PRESUBMIT.py
@@ -580,7 +580,7 @@
   """
   with _AddToPath(input_api.os_path.join(
       input_api.PresubmitLocalPath(), 'tools_webrtc', 'presubmit_checks_lib')):
-    from gn_check import RunGnCheck
+    from build_helpers import RunGnCheck
   errors = RunGnCheck(FindSrcDirPath(input_api.PresubmitLocalPath()))[:5]
   if errors:
     return [output_api.PresubmitPromptWarning(