swift234

Dependencies: Add new options to qualified lookup for known dependencies.

Specifically, a qualified lookup can now be treated as:
- a known private dependency, which does not affect downstream files
- a known non-private dependency, which may affect downstream files
- a known non-dependency
- unknown, which means the compiler will try to infer whether it's a
  private dependency from the context

This commit also includes some obvious uses of the new flags, but nothing
actually too interesting yet; there shouldn't be any observable behavior
change here in normal user code.

Swift SVN r23483