[Analyzer] Replace CXXSelfAssignmentBRVisitor with NoteTags
The cplusplus.SelfAssignment checker has a visitor that is added
to every BugReport to mark the to branch of the self assignment
operator with e.g. rhs == *this and rhs != *this. With the new
NoteTag feature this visitor is not needed anymore. Instead the
checker itself marks the two branches using the NoteTags.
Differential Revision: https://reviews.llvm.org/D62479