Use new Severity class and update *_warn_patterns

* This new class definition and patterns are
  shared between Android and ChromeOS compiler tools.
* Suppress hard to fix and false positive linter warnings.

Test: warn.py --url=http://cs/android --separator='?l=' build.log > warnings.html
Test: warn.py --gencsv build.log > warnings.csv
Change-Id: Icb47809100ad30796cb1da82610e989d450194fa

tools/warn/warn.py

@@ -19,6 +19,7 @@
 import multiprocessing
 
 # pylint:disable=relative-beyond-top-level
+# pylint:disable=g-importing-member
 from .warn_common import common_main