Refactor BugReporter interface to have a new 'BugReporterContext' and

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.

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@71121 91177308-0d34-0410-b5e6-96231b3b80d8