Sink private-file import support down to SerializedASTFile

Implementing it in LoadedFile is nice in theory, but causes a leak in
practice because that type is ASTContext-allocated and usually never
destroyed.

https://bugs.swift.org/browse/SR-11366

include/swift/Serialization/SerializedModuleLoader.h

@@ -274,6 +274,9 @@ public:
                            DeclName name, NLKind lookupKind,
                            SmallVectorImpl<ValueDecl*> &results) const override;
 
+  virtual StringRef
+  getFilenameForPrivateDecl(const ValueDecl *decl) const override;
+
   virtual TypeDecl *lookupLocalType(StringRef MangledName) const override;
   
   virtual OpaqueTypeDecl *