[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.

include/swift/AST/ASTContext.h

@@ -254,11 +254,6 @@ public:
   /// The request-evaluator that is used to process various requests.
   Evaluator evaluator;
 
-  /// The set of top-level modules we have loaded.
-  /// This map is used for iteration, therefore it's a MapVector and not a
-  /// DenseMap.
-  llvm::MapVector<Identifier, ModuleDecl*> LoadedModules;
-
   /// The builtin module.
   ModuleDecl * const TheBuiltinModule;
 
@@ -787,8 +782,24 @@ public:
   /// The loader is owned by the AST context.
   ClangModuleLoader *getDWARFModuleLoader() const;
 
+public:
   namelookup::ImportCache &getImportCache() const;
 
+  /// Returns an iterator over the modules that are known by this context
+  /// to be loaded.
+  ///
+  /// Iteration order is guaranteed to match the order in which
+  /// \c addLoadedModule was called to register the loaded module
+  /// with this context.
+  iterator_range<llvm::MapVector<Identifier, ModuleDecl *>::const_iterator>
+  getLoadedModules() const;
+
+  /// Returns the number of loaded modules known by this context to be loaded.
+  unsigned getNumLoadedModules() const {
+    auto eltRange = getLoadedModules();
+    return std::distance(eltRange.begin(), eltRange.end());
+  }
+
   /// Asks every module loader to verify the ASTs it has loaded.
   ///
   /// Does nothing in non-asserts (NDEBUG) builds.
@@ -828,6 +839,11 @@ public:
     return const_cast<ASTContext *>(this)->getStdlibModule(false);
   }
 
+  /// Insert an externally-sourced module into the set of known loaded modules
+  /// in this context.
+  void addLoadedModule(ModuleDecl *M);
+
+public:
   /// Retrieve the current generation number, which reflects the
   /// number of times a module import has caused mass invalidation of
   /// lookup tables.