Dependencies: member lookups in constraint systems are usually private.

This adds a new parameter to TypeChecker::lookupMember to specify when
something is known-private, which can then be passed along to
DeclContext::lookupQualified. This makes many of the existing member
lookup dependencies in the reference-dependencies.swift test correctly
count as private.

Swift SVN r23631

lib/Sema/ConstraintSystem.cpp

@@ -199,7 +199,8 @@ LookupResult &ConstraintSystem::lookupMember(Type base, DeclName name) {
 
   // Lookup the member.
   MemberLookups[{base, name}] = None;
-  auto lookup = TC.lookupMember(base, name, DC);
+  auto lookup = TC.lookupMember(base, name, DC,
+                                /*knownPrivate=*/isa<AbstractFunctionDecl>(DC));
   auto &result = MemberLookups[{base, name}];
   result = std::move(lookup);