Improved error reporting in IdempotentOperationChecker

- SourceRange highlighting is only given for the relevant side of the operator (assignments give both)
- Added PostVisitBinaryOperator hook to retrieve the ExplodedNode for an operator
- Added a BugReporterVisitor to display the last store to every VarDecl in a Stmt
- Changed bug reporting to use the new BugReporterVisitor

llvm-svn: 112839