swift234

[Driver] Refactor DependencyGraph in preparation for private dependencies.

- Give loadWithPath an enum result that includes "NeedsRebuilding".
  This will be returned when a new dependency is discovered that
  retroactively affects the graph.
- Don't clear the "provides" set for a node when it gets reloaded;
  just append to it. This lets us avoid calling markTransitive twice.
- Use proper types for "depends" and "provides" entries instead of std::pair.
- Use swift::OptionSet instead of a manual bitmask.
- Use separate "depends" and "provides" callbacks when parsing dependency
  files.

No expected functionality change.

Swift SVN r23851