[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 `NoteTag`s.

Differential Revision: https://reviews.llvm.org/D62479

llvm-svn: 361818