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

lib/Serialization/ModuleFile.cpp

@@ -1010,7 +1010,8 @@ void ModuleFile::getImportDecls(SmallVectorImpl<Decl *> &Results) {
         } else {
           SmallVector<ValueDecl *, 8> Decls;
           M->lookupQualified(ModuleType::get(M), ScopeID,
-                             NL_QualifiedDefault, nullptr, Decls);
+                             NL_QualifiedDefault | NL_KnownNoDependency,
+                             nullptr, Decls);
           Optional<ImportKind> FoundKind = ImportDecl::findBestImportKind(Decls);
           assert(FoundKind.hasValue() &&
                  "deserialized imports should not be ambigous");