swift234

Dependencies: start tracking whether a lookup is private to a file or not.

This is sort of two commits squashed into one: first, update
ReferencedNameTracker to record whether a name or type is non-private,
along with changing all clients to assume non-private; and second,
actually try to (conservatively) decide if a particular unqualified lookup can
be considered private.

What does "private" mean? That means that a dependency does not affect
"downstream" files. For example, if file A depends on B, and B depends on C,
then a change in C normally means A will get rebuilt. But if B's dependencies
on C are all private dependencies (e.g. lookups from within function bodies),
then A does not need to be rebuilt.

In practice there are several rules about when we can make this assumption,
and a few places where our current DeclContext model is not good enough to
distinguish private uses from non-private uses. In these cases we have to
be conservative and assume that the use is non-private (and thus that
downstream files will need to be rebuilt).

Part of rdar://problem/15353101

Swift SVN r23447