swift234

Add a flag to UnqualifiedLookup to say that a lookup is known-private.

...and thus does not affect downstream files...

...and adopt it in several places:
- when looking up the default type for a literal (test included)
- when looking up the first component in an IdentTypeRepr (test included)
- when deciding which ~= to use in a switch (test forthcoming)
- when a protocol has an operator function requirement (test forthcoming)
- when validating @NSApplicationMain and @UIApplicationMain
- when an enum element shows up unqualified in a switch
- several places where it doesn't matter because we're looking something up
  in the standard library.

Part of rdar://problem/15353101

Swift SVN r23670