travis: replace kernel check with clang check.

Check clang instead of kernel, which is a lot faster.
Make clang check failure fatal to the build.
travis: add -DCHECK_INTERNAL to the VERIFY=1 job to automatically fetch issues like #5869 via travis, check clang with --exception-handling
travis: make clang check sensitive to internal cppcheck errors.
travis: update clang repo hash
travis: suppress current warnings, make cppcheck not crash the files in clang that crash currently.
Merge remote-tracking branch 'matthiaskrgr/travis'