[ubsan] Add diagnostic handlers for nullability errors

Add 'nullability_arg' and 'nullability_return' diagnostic handlers, and
also add a TypeCheckKind for null assignments to _Nonnull. With this in
place, we can update clang to use the nicer handlers for nullability
diagnostics.

The alternative to this approach is to update the existing 'nonnull_arg'
and 'nonnull_return' handlers to accept a boolean parameter. However,
versioning the existing handlers would cause code size bloat, and the
complexity cost of introducing new handlers into the runtime is low.

I will add tests for this, and all of -fsanitize=nullability, into
check-ubsan once the clang side of the changes is in.

llvm-svn: 297748