BugReporter:

- Group control flow and event PathDiagnosticPieces into PathDiagnosticMacroPieces.
- Afterwards, eliminate any PathDiagnosticMacroPieces from a PathDiagnostic that
  contain no informative events.

HTMLDiagnostics:
- Use new information about PathDiagnosticMacroPieces to specially format
  message bubbles for macro expansions containing interesting events.

llvm-svn: 66524