Static Analyzer Diagnostics: Move custom diagnostic visitors from BugReporterContext to BugReport.

One API change: I added BugReporter as an additional parameter to the BugReporterVisitor::VisitNode() method to allow visitors register other visitors with the report on the fly (while processing a node). This functionality is used by NilReceiverVisitor, which registers TrackNullOrUndefValue when the receiver is null.

llvm-svn: 138001