BugReporter:

- Added an internal helper class 'PathDiagnosticBuilder' which now bundles the
  'ExecutionContinues' methods.
- Added preliminary diagnostics for short-circuit '&&' and '||'


git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@67822 91177308-0d34-0410-b5e6-96231b3b80d8