[analyzer] Fix null pointer deref in CastValueChecker

A crash was seen in CastValueChecker due to a null pointer dereference.

The fix uses QualType::getAsString to avoid the null dereference
when a CXXRecordDecl cannot be obtained. A small reproducer is added,
and cast value notes LITs are updated for the new debug messages.

Reviewed By: steakhal

Differential Revision: https://reviews.llvm.org/D127105