Wdeprecated: BugReporterVisitors are copied for cloning...

Wdeprecated: BugReporterVisitors are copied for cloning (BugReporterVisitorImpl), make sure such copies are safe

Make the copy/move ctors defaulted in the base class and make the
derived classes final to avoid any intermediate hierarchy slicing if
these types were further derived.

llvm-svn: 244979