Basic implementation of lookupVisibleDecls() for serialized modules.

This involved threading it through ModuleLoader, as with all the other
module-generic callbacks. I plan to collapse a bit of the chaining, but
unfortunately not that much.

This brings back the CodeCompletion tests.

Swift SVN r6527

lib/Serialization/ModuleFile.cpp

@@ -14,6 +14,7 @@
 #include "ModuleFormat.h"
 #include "swift/AST/AST.h"
 #include "swift/AST/ModuleLoader.h"
+#include "swift/AST/NameLookup.h"
 #include "swift/Basic/STLExtras.h"
 #include "swift/Serialization/BCReadingExtras.h"
 #include "llvm/Support/MemoryBuffer.h"
@@ -2213,3 +2214,13 @@ void ModuleFile::getReexportedModules(
     results.push_back({Module::AccessPathTy(), dep.Mod});
 }
 
+void ModuleFile::lookupVisibleDecls(Module::AccessPathTy accessPath,
+                                    VisibleDeclConsumer &consumer,
+                                    NLKind lookupKind) {
+  // FIXME: Validate against access path.
+  for (auto &topLevelEntry : TopLevelDecls) {
+    for (auto &value : topLevelEntry.second)
+      consumer.foundDecl(value);
+  }
+}
+