Fixed wrong configuration of itoa in std.cfg
Several improvements to CheckUnusedVar::checkFunctionVariableUsage_iterateScopes():
- Use AST in some places
- Fixed misusage of Token::isStandardType (fixes false negative)
- Removed some redundant conditions
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.