[ubsan] Use the nicer nullability diagnostic handlers

This is a follow-up to r297700 (Add a nullability sanitizer).

It addresses some FIXME's re: using nullability-specific diagnostic
handlers from compiler-rt, now that the necessary handlers exist.

check-ubsan test updates to follow.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@297750 91177308-0d34-0410-b5e6-96231b3b80d8