Refactor BugReporter interface to have a new 'BugReporterContext' and

'BugReporterVisitor'. This simplifies callbacks from BugReporter to BugReports
(via VisitNode). It also lays the foundation for arbitrary visitor "call backs"
that can be registered to a BugReporterContext as a PathDiagnostic is
constructed. These call backs can help operate as separate "experts" that can
work on constructed pieces of a PathDiagnostic for which they possess special
knowledge.

llvm-svn: 71121