[clang-tidy] Don't leak the TodoCommentHandler object

Preprocessor:addCommentHandler() does not take ownership,
so we'd end up leaking the TodoCommentHandler.

This patch makes it owned by the Check object.

Differential Revision: http://reviews.llvm.org/D5402
tsan: fix signal_longjmp test in debug mode
In debug mode tsan checks that user accesses
access user memory. NULL is not user memory.
So the test fails. Allocate real inaccessible
memory for the test.