[NFC] Hide LoadedModules From Clients of ASTContext

There's no reason clients need to be able to access this data directly.
It obscures where module loading is actually happening, and makes it too
easy to accidentally register a module with the wrong identifier in the
context.

Hide the registration operations behind opaque accessors.

lib/Serialization/SerializedSILLoader.cpp

@@ -27,7 +27,7 @@ SerializedSILLoader::SerializedSILLoader(
 
   // Get a list of SerializedModules from ASTContext.
   // FIXME: Iterating over LoadedModules is not a good way to do this.
-  for (auto &Entry : Ctx.LoadedModules) {
+  for (const auto &Entry : Ctx.getLoadedModules()) {
     for (auto File : Entry.second->getFiles()) {
       if (auto LoadedAST = dyn_cast<SerializedASTFile>(File)) {
         auto Des = new SILDeserializer(&LoadedAST->File, *SILMod, callbacks);